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RESEARCH LIBRARY 
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JOHN MOORE ANDREAS COLOR CHEMISTRY LIBRARY FOUNDATION 


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Elementary 
Photographic 
Chemistry 


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Eastman Kodak Company 
Rochester, N. Y. 
1928 


INTRODUCTION 


Photography is so essentially a chemical process that 
every photographer should have an interest in the chemicals 
which he uses and in the reactions which they undergo. 


This book is written in response to a demand for a simple 
account of photographic chemistry, for the practical photog- 
rapher. 

No attempt has been made to give the chemical theory 
in full, for which textbooks on chemistry should be con- 
sulted. In Chapter I, a statement is given only of the 
chemistry which is necessary to an understanding of the 
remainder of the book. In the same way reference should 
be made to photographic textbooks for general photographic 
practice. 

To give the information about photographic chemicals 
' which is necessary for their intelligent use, the properties 
of each of the more important chemicals are given in a separ- 
ate paragraph, which is inserted in the section dealing with | 
its use, in a distinct type face to facilitate reference. A table 
of solubilities of the chemicals in common photographic use 
is given at the end of the book. Practical information on 
formulas, preparing and using photographic solutions is given 
in Chapters VIII, IX and X. 

No apology is necessary for the insistence placed on the 
need for pure chemicals and on the advantage to be gained 
by using the Eastman Tested Chemicals, which are specially 
purified and tested for photographic use. 


EASTMAN KODAK COMPANY, 
Rocu_estTEr, N. Y. 
October, 1928. 


CONTENTS 


Chapter Page 
I. An Outline of Elementary Chemistry. . . . § 

II. The Chemistry of Photographic Materials . . 12 
III. The Chemistry of Development | . 2 oes 
IV. The Chemistry of Fixation . | 3 
V. The Chemistry of Washing —. >) 5 
VI. The Chemistry of Reduction and Intensification 36 
VII. The Chemistry of Toning . ~~.) 33) 32 eee 
VIII. Formulas... .. ....) . 
IX. Preparing Solutions . . . 2 =u 
X. Using Solutions”. =. 2 1) 


Table of Chemical Solubilities . 2) yeas 


CHAPTER I. 


An Outline of Elementary Chemistry 


All substances are made by the combination in various 
proportions of a limited number of elements, of which about 
ninety exist. These elements combine in definite proportions 
to form bodies of fixed composition, which are termed com- 
pounds. ‘Thus, one volume of the gaseous element hydrogen 
combines with one volume of the gaseous element chlorine to 
form two volumes of the compound hydrochloric acid gas. 
This combination can be represented by what is called a 
chemical equation. If we write H for hydrogen, C7 for chlo- 
rine and HC7/ for hydrochloric acid, we can represent the 
above combination by the equation 


H + Cl == H Cl 
Hydrogen Chlorine Hydrochloric Acid Gas 


It will be seen that an equation such as that given above is 
really a shorthand method of stating what happens, the ele- 
ments which take part in the combination being designated by 
letters. These letters which stand for the elements are called 
the “‘symbols” of the elements. 


The elements which are of the greatest importance in 
photography and their symbols are as follows: 


Gases 
Name Symbol Remarks 

Hydrogen H The lightest gas known. 

Nitrogen N Forms 80% of the air. (Approx.) 

Oxygen O Forms 20% of the air. (Approx.) 

. Chlorine CI Greenish-yellow poisonous gas. 

Bromine Br Poisonous brownish-red gas at high tem- 
peratures, liquid at ordinary tempera- 
tures. 


Non-metallic Solids 


Name Symbol Remarks 
Carbon ae Occurs in three forms: diamond, graphite, 
and charcoal or amorphous carbon. 
Sulphur S Yellowish-white, brittle solid. 
Todine I Violet plate-like crystals, similar in chem- 


ical properties to chlorine and bromine. 


ui 


6 EASTMAN KODAK COMPANY 


Name 
Sodium 


Potassium 


Calcium 
Aluminum 
Iron 


Copper 
Silver 
Platinum 
Gold 
Mercury 


Metallic Solids 


Symbol 
Na 


K 


Ca 
Al 
Fe 


Cu 
Ag 
Pt 
Au 
Hg 


Remarks 

Very light, attacked by moisture, kept 
under light oil. 

Very light, attacked by moisture, kept 
under light oil. 

Silvery white metal, attacked by moisture. 

Very light, white metal. 

In the pure state it is called wrought-iron; 
when containing a small amount of 
carbon it forms cast-iron and steel. 

Reddish, tough metal. 

White metal. 

Valuable white metal, very heavy. 

Reddish yellow metal, very heavy. 

White metallic liquid, very heavy. 


These elements fall into two groups; those which are 
metals and those which are not metals. Apart from the ap- 
pearance of the elements, the classification of an element in 
one of these two groups depends upon its relation to oxygen. 
Many of the elements when heated in the presence of oxygen 
will combine with it and will form what are called oxides. 
Thus, carbon will burn in oxygen and will form a gaseous 
compound of carbon with oxygen called carbon dioxide. Iron 
will burn in oxygen and forms a solid iron oxide. 


Name 
Hydrogen oxide 
(water) 


Carbon dioxide 
Nitric oxide 


Sulphur dioxide 


Aluminum oxide 


Calcium oxide 
Tron oxide 
Mercuric oxide 


Oxides of Elements 


Symbol 


H,O 


CO2 
NO 


See 


Al,O3 


CaO 
Fe,03 
HgO 


Remarks 
Can be made by burning hydrogen in air — 
or oxygen. 


Acid Oxides 


A heavy gas, is produced by burning car- 
bon; e.g., charcoal. 

Colorless gas, turns reddish-brown in con- 
tact with oxygen. 

Colorless gas. Produced by burning sul- 
phur. 


Basic Oxides 


White powder formed when aluminum is 
burned in the air. 

Quicklime, obtained by heating chalk. 

Red powder formed when iron rusts. 

Red powder formed by slow heating of 
mercury in the air. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 7 


Many oxides are soluble in water, forming two classes of 
compounds, which are known respectively as acids and bases, 
the acid oxides being produced from the non-metallic ele- 
ments and the basic oxides from the metallic elements. Thus, 
carbon, nitrogen and sulphur all form acid oxides which dis- 
solve in water to form acids, while sodium, potassium and 
calcium form typical basic oxides which dissolve in water to 
form bases. 


Bases are either alkaline or earthy, the alkaline bases 
being soluble, the earthy bases insoluble. The ordinary way 
of distinguishing between an acid and a base is to test the 
solution with a trace of certain dyes which change color 
according to whether the solution is acid or alkaline. Thus, 
if a piece of paper soaked in a solution of litmus, generally 
known as litmus paper, is put into a solution, it will turn red 
if the solution is acid, and blue if the solution is alkaline. 
Sodium forms an oxide which dissolves in water and makes a 
solution of basic caustic soda, the caustic soda having the 
formula NaOH, and being composed of sodium, oxygen and 
hydrogen. On the other hand, sulphur combines with oxygen 
and the oxide dissolves in water to form sulphurous acid, 
this having the formula H2zSO; and being formed by the 
combination of water, H2O, with sulphur dioxide, SO2z. Thus: 


SO2 + H20 — H2SO3 
Sulphur Dioxide Water = Sulphurous Acid 


All acids contain hydrogen and this hydrogen can be re- 
placed by a metal, forming a compound which 1s termed a 
“salt.” Thus, if we have sulphuric acid and we dissolve a 
piece of iron in it, the iron will replace the hydrogen of the 
acid, which will be given off as bubbles of gas and a solution 
of the salt, iron sulphate, will be formed: 


H2SO4 + Fe = FeSOa + H2 
Sulphuric Acid _— Iron Iron Sulphate | Hydrogen Gas 


Salts are also formed by the direct union of an acid and a 
base. Thus, if we have caustic soda, NaOH, and sulphurous 
acid, H2SOs, they combine to form sodium sulphite, eliminat- 
ing water. Thus: 


2Na0H + H2SO3 —— Na2zSO3 + 2H20 
Two partsof Sulphurous Acid Sodium Sulphite Water 
Caustic Soda 


8 EASTMAN KODAK COMPANY 


{t will be seen that the sodium sulphite is formed by the com- 
bination of the base derived from sodium with the acid de- 
rived from sulphur. 


Sometimes a non-metallic element forms two. different 
oxides, and these in turn will form two different acids. When 
we burn sulphur in oxygen, for instance, each atom of sulphur 
combines with two atoms of oxygen and forms sulphur di- 
oxide: | 


Ss + 20 = SO2 


and this dissolves in water to form sulphurous acid. If the 
sulphur dioxide is passed, with more oxygen over heated 
platinum, it is possible to make it combine with another atom 
of oxygen and form the compound sulphur trioxide, SOs, and 
this dissolves in water and forms sulphuric acid: 


SO3 + H20 — H2S04 


so that from sulphur we not only get sulphurous acid but a 
second acid—sulphuric acid. 


Just as the hydrogen of sulphurous acid is replaced by 
sodium to form sodium sulphite, so the hydrogen of sulphuric 
acid is replaced by sodium to form sodium sulphate. ; 


Sulphur Dioxide SO2 Sulphur Trioxide SO3 
Sulphurous Acid H2SO3 Sulphuric Acid H2S0O4 
Sodium Sulphite Na2SO3 Sodium Sulphate Na2SO4 


Salts are usually neutral to litmus paper, though some- 
times they are somewhat acid or alkaline. But in addition to 
the neutral salts, an acid in which there are two hydrogen 
atoms can have one of them replaced by a metal instead of 
both, and in this case we get acid salts, which are equivalent 
in their behavior to a mixture of equal parts of the acid and 
the neutral salt. For instance, from sulphurous acid if we 
replace both the hydrogens, we get sodium sulphite—NazSO; 
—but if we replace only one of the hydrogens, we get the - 
compound NaHSOs, which is called sodium acid sulphite, 
re hydrogen sulphite or, more usually, sodium bisul- 
phite. 

Sulphur forms a number of different acids. It forms not | 
only acids from its two oxides SOz and SOs, but it forms com- 
pound acids containing more than one atom of sulphur, and 
of these, one is of very great importance to the photographer, 
namely, thiosulphuric acid, which forms a sodium salt, sodium 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 9 


thiosulphate, NazS2O3. It will be seen that this compound 
differs from sodium sulphite in having two atoms of sulphur 
instead of one, and it is the compound, generally known as 
“hypo,” which is used for fixing photographi¢ materials. 


Some acids are formed not from oxides but by the direct 
combination of a non-metallic element with hydrogen, and of 
these the most important are the strong acids formed from 
chlorine, bromine and iodine, which three elements, because 
they occur in sea salt, are called halogens, from the Greek 
words, salt producing. Thus, chlorine combines directly with 
hydrogen to form hydrochloric acid, H Cl, and if the hydrogen 
of this is replaced by metals, we get chlorides, of which the 
best known is sodium chloride, Na Cl, which is common salt. 
Similarly, bromine combines with hydrogen to form hydro- 
bromic acid, with which metals form bromides, and in the 
same way the iodides are formed from iodine. 


The Halogens, Their Acids and Salts 


Halogen Element Acid Sodium Salt 
Cl Chlorine HCl Hydrochloric Acid NaCl Sodium Chloride 
Br Bromine HBr Hydrobromic Acid NaBr Sodium Bromide 
I Iodine HI Hydriodic Acid Nal Sodium Iodide 


Salts are soluble in water. to different extents, the solubil- 
ity depending upon the nature of the salt. Some, such as 
hypo, are extremely soluble, hypo being soluble in less than 
its own volume of water; while others are only slightly soluble 
or even almost completely insoluble, silver chloride, bromide 
and iodide being well known examples of very insoluble ma- 
terials. A solution of a salt may be regarded as containing 
both the basic and the acid components of the salt in a more 
or less free condition. For instance, all copper salts in solu- 
tion behave in much the same way, showing properties in 
common, due to the presence of the copper. In the same way 
all chlorides or sulphates show common properties in solution. 


; Now, when we mix two solutions of soluble salts, and the 

base of one can form an insoluble salt with the acid of the 
other, then this rearrangement will take place and the in- 
soluble substance will be thrown out of solution as a precipi- 
tate. Thus, silver nitrate and sodium chloride are both very 
soluble in water, but when the solutions are mixed, the silver 
and the sodium change places so that silver chloride and 
sodium nitrate are formed, and the almost insoluble silver 


fe) EASTMAN KODAK COMPANY 


chloride is thrown out of the solution, leaving only the sodium 
nitrate behind. 


AgNOs + NaCl. = £AgCl © + #£NaNOs 
Silver Nitrate Sodium Chloride Silver Chloride Sodium Nitrate 
Soluble Soluble Insoluble Soluble 


Precipitated 


This ‘double decomposition” is the simplest kind of chemical 
reaction and is the one with which we are most familiar. 


Other types of chemical reaction which are of great im- 
portance in photography are those of oxidation and reduction. 
The simplest example of oxidation is, of course, that in which 
an element combines with oxygen; but when an element forms 
two or more compounds with oxygen, then we are said to per- 
form oxidation when we raise the element from the level of 
oxidation of one of its compounds to another level in which 
it is combined with more oxygen. For example, by the oxida- 
tion of sodium sulphite, NazSOs, which is a compound formed 
from sulphur dioxide, SO2z, we get sodium sulphate NazSO,, 
which is derived from sulphur trioxide, SOs. This can be 
done by means of oxygen. If we pass air, which contains 20% 
of oxygen (the rest being chiefly nitrogen), through a sulphite 
solution, or even leave sodium sulphite exposed to the air for 
long periods, it will be oxidized into sulphate— 


NazSOs + O = NazSO4 
Sodium Sulphite | Oxygen — Sodium Sulphate 


When metallic elements form two oxides with different 
amounts of oxygen, these two oxides will act as bases for two 
series of salts. Thus, iron forms 


Ferrous salts derived from Fe O, and 
Ferric salts derived from Fe2Oz. 


Thus, we have 


Ferrous Chloride, Fe Clo, green crystals, 
Ferric Chloride, Fe Cl3, red-brown crystals. 


Very often oxidation is accomplished not by the use of 
oxygen itself but by the use of some substance which itself 
is a higher compound of oxygen and which can be reduced to 
a lower compound of oxygen or to an element which contains 
no oxygen at all. Thus, for instance, when hydroquinone is 
oxidized, we get quinone, which we call the oxidation product 
of hydroquinone, but if we add sulphite to quinone, the 
quinone oxidizes the sulphite to sulphate and is itself reduced 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY = 11 


again to hydroquinone. In this case the sulphite acts as a 
reducing agent, reduction being the opposite to oxidation. 
Thus, a body which is easily oxidized will take the oxygen 
it needs from other substances and so acts as a reducing 
agent. Hydroquinone is oxidized to guinone, which is reduced 
by sulphite to hydroquinone. Conversely, the sulphite is 
oxidized by the quinone to sulphate. 


Similarly, if we add ferric salts to hydroquinone, they will 
oxidize it to guinone and will themselves be reduced to ferrous 
salts. 


The term reduction is applied especially to the liberation 
of metallic elements from their compounds. If we heat mer- 
curic oxide, the oxygen is driven off by the heat and the 
mercuric oxide is reduced to mercury. Generally, reduction 
cannot be accomplished by heat alone, and it is necessary to 
have some substance present which can be oxidized in order to 
reduce a compound. Thus, to reduce iron from its oxide, of 
which iron ore is chiefly composed, we heat it with charcoal or 
carbon, which is oxidized to form carbon dioxide and which 
reduces the iron oxide to metallic iron. 


Chemical compounds consist of five great classes: 


1. ACIDS, which are formed from non-metallic elements and which contain 
hydrogen replaceable by a metal; 

2. BASES, which are formed from the metallic elements, and which, when 
soluble in water, are called alkalis; 

3. SALTS, which are formed from the combination of an acid and a base; 

4. OXIDIZERS, which are substances containing an excess of oxygen and 
which can give up this oxygen to another compound; 

5. REDUCERS, which are greedy for oxygen and which take the oxygen 
away from any compound containing an available supply of it. 


ELEMENTS 


| 


METALS NON-METALS 


form Oxides 
with Oxygen with Oxygen 
form Oxides 
with Hydrogen 


Oxides with form Oxides with 
water form water form 
Bases Acids 


eee 


12 EASTMAN KODAK COMPANY 


CHAPTER II. 


The Chemistry of Photographic 
Materials 


The art of photography is founded upon the fact that the 
compounds of silver, and especially its compounds with chlo- 
rine, bromine and iodine, are sensitive to light. 


The earliest photographs were made by coating paper with 
silver chloride and using this to form images by its darkening 
under the action of light, but the sensitiveness of the silver 
chloride was too slight to use it in this way to form images 
in the camera. 


To get results which require less exposure to light, advan- 
tage is taken of the fact that it is not necessary for the light 
to do the whole work of forming the image; it is possible to 
expose the silver salt for only a short time to the light and 
then to continue the production of the image by chemical 
action, the process being termed “development.” 


Sensitive photographic materials therefore consist of 
paper, film, or glass coated with a layer in which is suspended 
the sensitive silver bromide or silver chloride. This layer is 
called the emulsion. This emulsion consists of a suspension of 
the silver salt in a solution of gelatin. It is made by soaking 
gelatin in water until it is swollen and then dissolving it by 
gently warming and stirring. The necessary bromide or chlo- 
ride, e.g., potassium bromide or sodium chloride, is then 
added to the solution and dissolves in it. Meanwhile, the 
right quantity of silver nitrate to react with the quantity of 
salts used has been weighed out and is dissolved in water. 
The silver nitrate solution is then added slowly to the solu- 
tion of gelatin and salt and produces in it a precipitate of the 
silver compound, the mixing being done in the darkroom, 
since the silver compound produced is sensitive to light. If 
there were no gelatin in the solution the silver compound 
would settle down to the bottom and an emulsion would not 
be formed, but the gelatin prevents the settling so that as the 
silver nitrate is added a little at a time, the precipitated silver 
salt is uniformly distributed through the solution. If this 
emulsion is coated on a support, such as paper or film and 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 13 


then cooled, the gelatin will set to a jelly, and when the jelly 
is dried we get a smooth coating of dry emulsion of the sensi- 
tive silver compound. 


Photographic materials which are to be developed must 
contain no excess of soluble silver and the emulsion must be 
made so that there is always an excess of bromide or chloride, 
since any excess of soluble silver will produce a heavy fog over 
the whole of the surface as soon as the material is placed in the 
developer. In the case of Solio paper, however, which is not 
used for development but which is printed out, a chloride 
emulsion is made with an excess of silver nitrate. This causes 
rapid darkening in the light, so that prints made upon Solio 
paper are not developed, the visible image being toned and 
fixed. Solio paper can be developed with certain precautions, 
such as by the use of acid developers or after treatment with 
bromide to remove the excess of silver nitrate. 


In the early days of photography prints were usually made 
on printing-out papers, but at the present time most prints 
are made by the use of developing-out chloride and bromide 
papers, which are chemically of the same nature as the nega- 
tive making materials and are coated with emulsions contain- 
ing no free silver nitrate. 


Negative making materials such as plates and films, al- 
ways contain silver bromide with a small addition of silver 
iodide. The different degrees of sensitiveness are obtained 
by varying the temperature and the duration of heating 
which the emulsions undergo during manufacture, the most 
sensitive emulsions being heated to higher temperatures and 
for a longer time than the slower emulsions. 7 


If a slow bromide emulsion is coated upon paper, the ma- 
terial is known as bromide paper and is used for printing, and 
especially for making enlargements. The less sensitive papers 
which are commonly used for contact printing by artificial 
light contain silver chloride in the place of silver bromide. 


In order to obtain silver nitrate the first step is to dissolve 
metallic silver in nitric acid. The silver replaces the hydrogen 
of the acid and forms silver nitrate, the hydrogen liberated 
decomposing a further portion of the nitric acid. The silver 
nitrate is crystallized out of the solution and obtained in color- 
less, transparent plates. 


SILVER NITRATE for photographic use has to be extremely pure, and 
since metallic silver usually contains a small quantity of other metals, such 
as copper and lead, it is necessary to free it from these impurities. This 1s 


t4 EASTMAN KODAK COMPANY 


accomplished by recrystallization, so that the silver nitrate is finally ob- 
tained in a perfectly pure form. 


In order to insure the purity of the silver nitrate which it uses, the East- 
man Kodak Company prepares its own and is the largest maker of silver 
nitrate in the world, using about one-eleventh of all the silver mined in the 
United States, the Mint only, using more. 


Silver nitrate is very soluble in water. It attacks organic material, and 
blackens skin, wood, cloth, and other similar substances on exposure to light. 


When a solution of silver nitrate is added to a solution of 

a bromide or chloride of another element, a reaction occurs 

are the insoluble silver bromide or chloride is precipitated. 

Thus, if we add silver nitrate to potassium bromide, the re- 
action occurs according to the following equation: 


Ag NO3 a K Br — Ags Br ~- -- KNOs3 
Silver Nitrate Potassium Bromide Silver Bromide Potassium Nitrate 


The potassium nitrate formed remains in solution, but if the 
solution is at all concentrated, the silver bromide is thrown 
down to the bottom of the vessel as a thick, curdy precipitate. 


The bromides and chlorides used in photography are 
chiefly the salts of potassium and sodium. Both the bromides 
and the chlorides are obtained from naturally occurring salt 
deposits, but, whereas these deposits consist chiefly of chlo- 
rides, they contain only a very small quantity of bromide, and 
bromide is therefore a very much more expensive material 
than chloride. 


The elements chlorine, bromine and iodine are all obtained 
from natural salt or from the sea, iodine being derived from 
certain sea weeds which extract it from the sea water and thus 
‘make it available in a concentrated form. Chlorine is a 
yellowish-green gas, very suffocating and poisonous, bromine 
gives dark red fumes which are even more noxious than 
chlorine and condense to a liquid, and iodine forms shining, 
black crystalline flakes which on heating give a violet vapor. 
The chief chlorides, bromides and iodides used in photog- 
raphy are the following: 


AMMONIUM CHLORIDE: White crystals soluble in water. Made 
from ammonia and hydrochloric acid, should have no smell, and when evapo- 
rated by heat should leave no residue behind. 


AMMONIUM BROMIDE: Very similar to the chloride, which is the only 
impurity likely to be present. 


AMMONIUM IODIDE: Should consist of colorless crystals. Decom- 
poses in light and is stained yellow by the iodine liberated. Very soluble in 
water and deliquescent (see p. 23). Soluble in alcohol. 


PEEMEN TARY PHOTOGRAPHIC CHEMISTRY 1s 


SODIUM CHLORIDE: Ordinary table salt is fairly pure sodium chlo- 
ride and a very pure salt is easily obtained. The pure salt is stable and not 
deliquescent. Soluble in cold water (40°F.) (4°C.) to the extent of 31 ounces 
of salt to 100 fluid ounces of solution. Solubility increases very little on 
heating. . 


SODIUM BROMIDE: A white salt, similar to the chloride but more 
soluble. Is generally pure but may contain chloride. 


POTASSIUM CHLORIDE: White salt, very similar to sodium chlo- 
ride. 


POTASSIUM BROMIDE: Occurs as colorless cubical crystals and is 
generally pure. To facilitate handling and weighing potassium bromide 
is usually supplied in the granular form. Very soluble in water. 


POTASSIUM IODIDE: Similar to bromide. Very soluble. May con- 
tain as impurities carbonate, sulphate and iodate, but is usually pure. A 
solution of potassium iodide dissolves iodine, which is insoluble in water, 
and it is therefore used to prepare a solution of iodine. 


The gelatin which is used to emulsify the sensitive silver 
salts is a very complex substance which is obtained from the 
bones and skins of animals. Gelatin has some curious and 
valuable properties. In cold water it does not dissolve but 
it swells as if, instead of the gelatin dissolving in the water, 
the water dissolves in the gelatin. If the water is heated, 
the gelatin will dissolve, and it can be dissolved to any 
extent. It cannot be said that there is a definite solubility of 
gelatin in water in the same sense as salts may be considered 
to have a definite solubility. As more gelatin is added, the 
solution becomes thicker. If the gelatin solution is heated, it 
will become thinner and less viscous when hot, and will 
thicken again as it cools, but it will remain thinner than if it 
had not been heated, so that the heating of the gelatin solu- 
tion produces a permanent change in its properties. If a 
gelatin solution is cooled, the gelatin will not separate from 
the solution in a dry state but the whole solution will set to 
a jelly. If the jelly is heated again, it will melt, and a jelly 
can be melted and reset many times. During the treatment 
there will be produced a progressive change in the jelly, and 
if the process 1s continued long enough, the solution will refuse 
to set and will remain as a thick liquid. 


Gelatin belongs to the class of substances which are called 
colloids, the name being derived from a Greek word meaning 
“gummy.” When a gelatin jelly is dried it shrinks and forms 
_ahorny or glassy layer of the gelatin itself, smooth and rather 
brittle. This dry gelatin, when placed in water, will at once 
absorb the water and swell up again to form a jelly. This 
swelling and shrinking are of great importance in photog- 


16 EASTMAN KODAK COMPANY 


raphy. When a photographic material with an emulsion made 
with gelatin is placed in water, the film will swell up and will 
continue to absorb more water and swell for a long time, fin- 
ally becoming soft and even dissolving, the extent to which 
this occurs depending on the temperature and the nature of 
the solutions in which it is placed. A small quantity of either 
an acid or alkali will produce a considerable increase in the 
swelling, and since the developer is alkaline and the fixing 
bath is acid, both these solutions have a great tendency to 
swell the gelatin, especially when they are warm. In order 
to avoid difficulty from this source, gelatin emulsions have 
a hardener added before they are coated, gelatin being hard- 
ened and made more resistant to swelling by the addition of 
alum. Ge 

Under ordinary circumstances no difficulty is experienced 
by the photographer due to the softening of the gelatin, but 
when photographic materials are exposed to extreme tem- 
peratures, care must be taken in handling them. Hardening 
agents such as alum must be added to the fixing bath, and 
all solutions must be kept at the same temperature in order 
to avoid sudden contractions or expansions of the gelatin 
which may result in detaching the film from its support or 
in the production of reticulation, i.e., a coarse wrinkling all 
over the film. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 1 7 


CHAPTER ITI. 


The Chemistry of Development 


When a light sensitive material is exposed for a short time 
to light, although the change which takes place may be so 
minute that it cannot be detected by any ordinary means, if 
the exposed material is placed in a chemical solution, which is 
termed the “developer,” the chlorine or bromine is taken 
away from the silver, and the black metallic silver which re- 
mains behind forms the image. This image is, of course, 
made up of grains, because the original emulsion contains the 
silver bromide in the form of microscopic crystals, and when 
the bromide is taken away from each of these, the crystal 
breaks up and a tiny coke-like mass of metallic silver remains 
behind in exactly the same position as the bromide crystal 
from which it was formed, so that, whereas the original emul- 
sion consisted of microscopic crystalline grains of the sensitive 
silver salt, the final image consists of equally microscopic 
grains of black metallic silver. This removal of the bromide 
from the metallic silver is known chemically as reduction. 
(It must be remembered that chemical reduction has nothing 
to do with the photographic operation known as the reducing 
of a negative; that is, the weakening of an over-dense nega- 
tive, where the word simply refers to the removal of the silver 
and is not used in the chemical sense.) 


Chemical reducers are substances which have an affinity 
for oxygen and which can liberate the metals from their salts, 
such as the charcoal which, as explained in Chapter I, is used 
to reduce iron from its ore. A developing solution is therefore 
one which contains a chemical reducer. All substances which 
are easily oxidized are, however, not developers, since in order 
that a reducer may be used as the photographic developer it 
is necessary that it should be able to reduce exposed silver 
bromide but should not affect unexposed silver bromide, so 
that its affinity for oxygen must be within certain narrow 
bounds; it must be a sufficiently strong reducer to reduce the 
exposed silver salt, and at the same time must not affect that 
which has not been exposed. For practical purposes the 
developing agents are limited to a very few substances, almost 
all of which are chemically derived from benzene, the light 
oil which is distilled from coal tar. 


18 EASTMAN KODAK COMPANY 


The commonest developing agents are pyrogallol (pyro), 
hydroquinone, Elon, para-aminophenol or Kodelon, and di- 
aminophenol. 


PYROGALLOL (or pyrogallic acid) is made from gallic acid, which 
is obtained from gall nuts imported from China. The gall nuts are fermented 
to obtain the gallic acid, and the gallic acid is then heated in a still from 
which the pyrogallol is distilled over. Pyrogallol is made in two forms: a 
flaky powder form and a crystal form. When the powdered pyrogallol is 
opened in the darkroom or studio, the fine particles fly about and are likely 
to settle on paper or plates, producing spots on the photographs. For this 
reason the Eastman Kodak Company manufactures and supplies pyrogallol 
in the crystal form, which can be handled without any danger of particles 
flying about and giving trouble. 


HYDROQUINONE is made from benzene which is first converted into 
aniline and then oxidized. Although it is somewhat less powerful as a re- 
ducing agent than pyro, it has less propensity to give stain and when used 
in conjunction with Elon or Kodelon it is a very useful developer, in fact, 
it is a constituent of a majority of the better known commercial developers 
in use today. It keeps very well when used in tank developers because it 
does not oxidize as readily as pyro and is generally used in motion picture 
work. Its purity is very important and Eastman Tested Hydroquinone, 
manufactured by the Company, may be relied upon for all formulas. 


Some time after pyrogallic acid and hydroquinone were in 
general use by photographers, there were introduced a num- 
ber of new developing agents made from coal tar, which are 
very useful as supplements to the older developers. Several 
of these are based on a substance called para-aminophenol, 
which is made in the manufacture of dyes. When para- 
aminophenol is treated with methyl alcohol the methyl part 
of the alcohol attaches itself to it and forms a compound called 
methyl-para-aminophenol, which is a more active developing 
agent than the para-aminophenol itself. Another developing 
agent of the same type is dtaminophenol, which is prepared in 
a way similar to para-aminophenol. 


Para-aminophenol, methyl-para-aminophenol and _ di- 
aminophenol are all bases and the developing agents are their 
salts, the oxalate of para-aminophenol, the hydrochloride of 
diaminophenol being used, and the sulphate of methyl-para- 
aminophenol. 


Para-AMINOPHENOL OXALATE is manufactured and sold by the 
Eastman Kodak Company under the name of Kodelon. Many of the so- 
called ‘‘new” developing agents on the market consist entirely or mainly 
of para-aminophenol. A good sample should be light in color and should 
burn entirely when heated to redness, leaving no ash behind. 


MONOMETHYL Para-AMINOPHENOL SULPHATE manufactured 
and sold by the Eastman Kodak Company under the name of Elon. Mono- 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 1g 


methyl para-aminophenol sulphate is distinguished sharply from para-amino- 
phenol oxalate by the fact that it is soluble in the cold in its own weight 
of strong hydrochloric acid, whereas the para-aminophenol oxalate is 
insoluble. 


DIAMINOPHENOL HYDROCHLORIDE is sold by the Eastman Kodak 
Company under the trade name of 4cro/. It is a steel gray powder, darken- 
ing easily in the air and is oxidized so rapidly in solution that it is usual to 
dissolve it only when required for use. 


Different reducing agents behave differently as develop- 
ers. We cannot use Elon in the place of hydroquinone and 
get the same effect. An image developed with Elon comes up 
very quickly and gains density slowly, while the hydroqui- 
none image comes up very slowly but gains density steadily 
and rapidly. A very little change in the temperature affects 
hydroquinone greatly and affects Elon very little, and in the 
same way a small quantity of sodium or potassium bromide 
affects hydroquinone and does not affect Elon nearly so 
much. ‘These differences in the developing agents depend 
upon the chemical nature of the substances themselves, and 
the particular property to which these differences are due is 
called the “reduction potential” of the developer. 


The reduction potential alone does not determine the 
speed with which the developer develops the image, because 
this depends chiefly upon the rate at which the developer dif- 
fuses into the film and on the quantity of developing agent 
and other substances in the developer. A high reduction 
potential enables a developer to continue to develop more 
nearly at a normal rate under adverse circumstances, such 
as at a low temperature or in the presence of bromide. The 
reduction potential of a developer, in fact, may be compared 
to the horse-power of an automobile which for other reasons 
than the power of its engine is limited in speed. If we have 
two automobiles and they are confined to a maximum speed 
of twenty miles an hour, then on level roads the one with the 
more powerful engine may be no faster than that with a 
weaker engine, but in a high wind or on a more hilly road the 
more powerful engine will allow the automobile to keep its 
speed, while the machine with the weaker engine will be 
forced to go more slowly. We could, indeed, measure the 
horse-power of an automobile by the maximum grade which 
it could climb at a uniform speed of 20 miles an hour. 

In development, the analogy to the hill is the addition of 
bromide to the developer, since the addition of bromide 
greatly retards development, and it is found that the higher 


20 EASTMAN KODAK COMPANY 


the reduction potential of a developer, the more bromide is 
required to produce a given effect. If we measure the de- 
veloping agents in this way, we shall find that hydroquinone 
has the lowest reduction potential, then Athenon, then pyro, 
then Kodelon, and finally Elon, which has the highest. Hy- 
droquinone has so low a potential that it is rarely used alone 
but is generally used with Elon. Kodelon can be substituted 
for Elon but more Kodelon has to be used in order to produce 
a developer of the same strength. Developers with a high 
reduction potential such as Elon, and to a less extent Kode- 
lon, make the image flash up all over at once, because they 
start development very quickly even in the lesser exposed 
portions of the emulsion, while developers of low reduction 
potential, like pyro and. especially hydroquinone, bring up 
the highlights of the image first and the shadows do not fully 
appear until the highlights are somewhat developed. 


Most developing agents cannot develop at all when used 
by themselves. With the exception of Acrol, developing 
agents, in order to do their work, must be in an alkaline solu- 
tion, and the energy depends upon the amount of alkali 
present. The developers of higher reduction potential, which 
bring up the image very quickly, require less alkali than those 
of lower reduction potential. For instance, hydroquinone is 
often used with caustic alkalis, while the other developing 
agents require only the weaker carbonated alkali. 


The quantity of alkali governs the energy of a developer, 
and if too much alkali is present, the developer will tend to 
produce chemical fog, while if too little alkali 1s present, it will 
be slow in its action. Alkalis also soften the gelatin of the 
emulsion, and consequently too alkaline a developer will pro- 
duce over-swelling and will give trouble with frilling or 
blisters in warm weather. 


The alkalis most commonly used in development are of 
two kinds: the caustic alkalis and the carbonated alkalis. 


Caustic alkalis are produced when the metal itself reacts 
with water, the metals from which the alkalis generally used 
are derived being potassium and sodium. These metals are so 
easily oxidized that they have to be preserved from all con- 
tact with air or water by immersion in light oil or gasoline. 


If we take a small piece of sodium and place it on the sur- 
face of water in a dish, it will react with the water with great 
violence, melting with the heat produced and sputtering 
about the surface; while if we restrict its movement, the 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 21 


development of heat will be so great that the hydrogen pro- 
duced will burst into flame. In the case of potassium, the 
reaction is even more violent than with sodium and is always 
accompanied by fame. The reaction may be represented by 
the equation— 


Na + H20. — NaOH + H 
Sodium Water Sodium Hydroxide Hydrogen 


the sodium combining with the water to form caustic soda 
and liberating hydrogen, which comes off as gas, and, as has 
already been stated, catches fire and burns in the air. This 
is, of course, not the method by which the alkalis are actually 
produced. As a matter of fact, the metals are produced by 
electroplating the metal out from the melted alkali. 


SODIUM HYDROXIDE (CAUSTIC SODA) is made either by the 
passage of an electric current through a solution of common salt, when the 
soda separates at one electrode and chlorine gas is liberated at the other, 
or from sodium carbonate, which is causticized by means of lime. Lime is 
calcium oxide and is prepared by heating limestone, which is calcium car- 
bonate, the carbon dioxide being driven off from the limestone by the heat. 
When the lime is added to sodium carbonate, the lime removes the carbon 
dioxide from the carbonate, and leaves the sodium hydrate in the solution, 
which is then evaporated to get the solid substance. At present, caustic soda 
is easily obtained in a very pure state, and there is usually no difficulty in 
getting good caustic soda for photographic work. It must be protected from 
the air, since it easily absorbs moisture and carbon dioxide. As its name 
indicates, it is very caustic and attacks the skin, clothing, etc. 


POTASSIUM HYDROXIDE (CAUSTIC POTASH) is very similar to 
caustic soda and is prepared in the same way. Fifty-six parts of caustic 
potash are chemically equivalent to forty parts of caustic soda. 


Both caustic soda and caustic potash should be dissolved in cold water 
when solutions of either chemical are prepared, because on mixing con- 
siderable heat is evolved and the solution, if too hot, is apt to boil and spatter 
on the hands or face causing serious burns. 


An alkali which was often used with pyrogallol in the early 
days of photography, but which is rarely used nowadays, 1s 
ammonia. Nitrogen combines with three times its volume 
of hydrogen to form a gas, NHs. This gas is known as am- 
monia and is very soluble in water, its solution being strongly 
alkaline. Ammonia combines directly with acids to form 
salts which are analogous to the salts of sodium and potas- 
sium. Thus with hydrochloric acid it forms ammonium 
chloride, which is similar to sodium chloride and potassium 


chloride: 


NH3 + HCl = NH4Cl 
Ammonia Hydrochloric Acid © Ammonium Chloride 


29 EASTMAN KODAK COMPANY 


Ammonia is a somewhat weaker alkali than soda or potash 
but stronger than the carbonates. For use in development it 
has the disadvantage that being used in the form of a solution 
of a gas its strength is somewhat uncertain and variable, the 
ammonia escaping from the solution. Also, it is a solvent of 
silver bromide and tends to produce colored stains which are 
not so easily produced with other alkalis. 


AMMONIA SOLUTION is commercially prepared from the ammoniacal 
liquor obtained in the distillation of coal for coal gas. The liquor is neutral- 
ized with sulphuric acid, the ammonium sulphate crystallized out, and the 
ammonia gas liberated from the sulphate with lime and led into water, in 
which it dissolves. The solution is usually free from impurities. 


Ammonia solutions are prepared commercially in two strengths, “am- 
monia water,” containing 10% of ammonia gas by weight and having a 
specific gravity of 0.96, and “stronger ammonia water” containing 28% of 
ammonia by weight and having a specific gravity of 0.90. 


The alkalis generally used for photographic work are not 
the caustic alkalis but the carbonates, which are salts of car- 
bonic acid, HzCO3. Carbonic acid is a very weak acid, so that 
in solution the carbonates are not neutral but alkaline because 
of the predominance of the strong base over the weak acid, the 
carbonate being, to some extent, split up into the bicarbonate 
or acid carbonate and the caustic alkali. The use of a carbon- 
ate in development therefore represents a sort of reservoir of 
alkali, only a small quantity of alkali being present at any 
time, but more being generated by dissociation of the car- 
bonate as itis used up. If instead of using carbonate we were 
to use for development a solution containing a proportional 
quantity of caustic alkali, we should have only a small 
quantity of alkali present, and it would soon be exhausted. 
The use of carbonate, therefore, enables us to employ a small 
concentration of alkali and yet to keep that concentration 
nearly constant during use. 


When a salt is dissolved in water at a high temperature un- 
til no more will dissolve and then the solution is allowed to 
cool, the salt will generally be deposited in crystals; some- 
times, as in the case of silver nitrate, the crystals consist of 
the pure substance, but more often each part of the salt 
combines with one or more parts of water to form the crystals. 
This combined water is called “water of crystallization.” 
Thus, crystals of sodium carbonate formed from a cool solu- 
tion contain ten parts of water to one of carbonate, and their 
composition should be written: 


Na2COz3 . 10H20 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 2 9 


What is called in the last paragraph a “‘part”’ of sodium car- 
bonate, NazCOs, will weigh 106 units, while a “‘part’”’ of water, 
HO, weighs 18 units, so that the crystals of sodium carbonate 
contain 106 parts by weight of sodium carbonate and 180 
by weight of water, and consequently crystallized sodium car- 
bonate contains only 37% of dry sodium carbonate. If 
sodium carbonate is crystallized from a hot solution only one 
part of water is combined in the crystals with each part of 
sodium carbonate so that they have the composition NazCOs . 
H20 and contain 85% of dry carbonate. Sodium carbonate 
containing ten parts of water of crystallization loses nine of 
them by drying in the air and,breaks up, forming the com- 
pound with one part of water. ris last part of water is only 
removed with difficulty by heating in the air, when the dry 
carbonate is formed, containing only a small residual quantity 
of water and about 98% carbonate. 


When exposed to the air chemicals often either absorb or 
give up water. Those which absorb water are said to be “‘hy- 
groscopic,” and if they absorb so much that they dissolve and 
form a solution they are said to be “‘deliquescent.”’ Chemicals 
which give up water to the air, so that the crystals break down 
and become covered with powder, are called ‘“‘efflorescent.”’ 


SODIUM CARBONATE comes on the market in three forms: Crystals 
with ten parts of water, NagCO3 . 10H2O containing 37% of the carbonate; 
crystals with one part of water, NagCO3 . H2O, containing 85% of the car- 
bonate, and the dry powder containing 98% of the carbonate. The carbon- 
ate is made by treatment of salt solution with ammonia and carbon dioxide 
which reacts with the salt to produce sodium bicarbonate, NaHCO3. The 
bicarbonate is heated and half of the carbonic acid driven off, producing 
crude sodium carbonate, which at this stage is known as “‘soda ash.”’ This 
is then dissolved in water, and crystals of “‘sal soda,’ containing ten parts 
of water, are produced. From this a crystalline salt with either one or ten 
parts of water is prepared for photographic use, but owing to the uncertainty 
of the composition of these crystals it is better to prepare the pure dry car- 
bonate. This is obtained by heating the pure bicarbonate which can be 
precipitated from a solution of sal soda by means of carbon dioxide gas. 
When the bicarbonate is heated in the air, half of the carbonic acid is driven 
off, and sodium carbonate, NagCOs, is produced according to the equation: 


2 NaHCO3 — Na2CO3 ++ CO2 + H20 
Sodium Bicarbonate Sodium Carbonate Carbon Dioxide Water 


The exact amount of heating is very important. If it is not done for suffi- 
cient time there will be a large quantity of bicarbonate left in the product, 
and bicarbonate is practically useless as an alkali in photography. On the 
other hand, if heating is continued too long, caustic soda will be produced. 
In the preparation of photographic carbonate the heating should be con- 
tinued so that the material is almost pure sodium carbonate containing prac- 


24 EASTMAN KODAK COMPANY 


tically no bicarbonate butis very slightly on the alkaline side. Much caustic soda 
would be fatal, but it is better to have a trace of caustic soda than bicarbon- 
ate. The preparation of sodium carbonate is a matter to which the greatest 
attention is given by the Eastman Kodak Company, and the E. K. Tested 
Carbonate is specially prepared to meet the-needs of the photographer. 


POTASSIUM CARBONATE is sometimes used instead of sodium car- 
bonate in developer formulas. Although it is more soluble than sodium 
carbonate, it has the disadvantages of being more expensive and absorbs 
water very readily. It must, therefore, be kept in well-sealed bottles. 


Another alkali which has come into extensive use recently 


is borax. This chemical is recommended for use in a de- 
veloper especially suited for the production of fine grained 
motion picture negatives. very photographic image is 
composed of tiny coke-like masses of silver which have been 
reduced from the original crystals of silver bromide in the 
sensitive emulsion. The advantage of keeping the particles 
as small as possible is obvious, especially in motion picture 
work where the individual pictures comprising a film are 
enlarged several hundred times during projection. One of 
the causes of “‘graininess’”’ or the coarsening of these tiny 
particles in the picture image is the fusion or clumping of 
the grains which occurs during development. Experiments 
have shown that several silver halide crystals in close proxim- 
ity to each other, even though unexposed, may become de- 
veloped and form a clump as a result of actual contact with 
an exposed crystal. | 


In the’ special developer (Formula D-76, page 52) there 
is a high concentration of sulphite which is a solvent for 
silver bromide and iodide. As development progresses 
therefore, the sulphite actually dissolves a small quantity of 
each grain and thereby minimizes greatly the tendency for 
clump formation which would increase the graininess. Addi- 
tion of carbonate to such a developer increases the rate of 
development and also accentuates the graininess of the re- 
sulting negative. 


BORAX or SODIUM TETRABORATE is prepared from certain calcium 
ores by first roasting, then boiling in sodium carbonate and bicarbonate 
solution, and finally crystallizing in large iron vats. A new source of borax 
discovered in Kern County, California in 1926 is virtually pure sodium borate 
and requires only dissolving, filtering, and recrystallizing to prepare it for the 
market. The pure salt forms large crystals readily soluble in hot water. It 
is used in developers for the production of fine grained negatives and in 
acid hardeners for prints which are to be dried on belt driers. 


Owing to the fact that developers are necessarily sub- 
stances which have a great affinity for oxygen and that the 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY — 25 


air contains oxygen, developing solutions containing only the 
developing agent and alkali would be rapidly spoiled from 
oxidation by the air. In order to make the developer keep 
there is added to the developing solution, in addition to the 
reducing agent and alkali, some sodium sulphite. Sodium 
sulphite has a very strong affinity for oxygen, being easily 
oxidized to sodium sulphate (see page 10), so that it protects 
the developer from the oxygen of the air, thus acting as a 
“preservative.” This action of the sulphite is very easily 
seen with the pyrogallol developer. The oxidation product 
of pyrogallol is yellow, and this oxidation product which is 
formed in development is deposited in the film along with 
the silver, so that if we use a pyrogallol developer without 
sulphite we shall get a very yellow negative, the image con- 
sisting partly of silver and partly of the oxidized pyrogallol. 
If we use sulphite in the developer, the image will be much 
less yellow because the pyrogallol will be prevented from 
oxidizing, the sulphite being oxidized instead, and finally if 
we add a great deal of sulphite, we shall get almost as blue an 
image as with Elon, the oxidation product of which is not 
deposited in a colored form with the silver. 


SODIUM SULPHITE is prepared by blowing sulphur dioxide gas into a 
solution of sodium carbonate. When sulphite is crystalized from the cooled 
solution it forms crystals containing seven parts of water to one of sulphite, 
of the composition Na,SO3 . 7H2O which contain, when pure, 50% of dry 
sulphite. These crystals give up water when kept in the air and form a white 
powder on the surface. Since sulphite, when exposed to the air, has a ten- 
dency to oxidize to the sulphate, and as the sulphate is not a preservative, it 
is well to view with suspicion sulphite which has efforesced to a great extent. 
A quick rinse in cold water will remove the white powder from the crystals. 


Sulphite free from water is produced by two methods: by drying the 
crystals, which produces what is called the “desiccated” salt, containing 
about 92% of pure sulphite, and by precipitation from hot solutions which 
gives a compound generally called “anhydrous” sulphite, and which con- 
tains as much as 96.5% of sulphite. 


Eastman Tested Sulphite is the desiccated salt, and is prepared in a pure 
state almost free from sulphate. If prepared in this way as a dry powder 
the sulphite will keep well for a long time. 


Sodium forms a number of compounds with sulphurous 
acid in addition to sodium sulphite itself. Thus there is 
sodium acid sulphite or bisulphite, NaHSOs, which may be 
regarded as a compound of sodium sulphite with sulphurous 
acid: 

Na2SO3 f- H2SO3 = 2 NaHSOs3 
Sodium Sulphite Sulphurous Acid Sodium Bisulphite 


526 EASTMAN KODAK COMPANY 


Another, sodium metabisulphite, 1s a compound of sodium 
sulphite with sulphur dioxide: 


Na2SO3 -+- SO2 = Na2S20s 
Sodium Sulphite | Sulphur Dioxide | Sodium Metabisulphite 


Ordinary commercial bisulphite has been shown by 
analysis to consist chiefly of metabisulphite which is con- 
verted into bisulphite when dissolved in water. Commer- 
cially dry sodium bisulphite is supplied by the Eastman 
Kodak Company in a very pure form as one of its Tested 
Chemicals. It may be used with entire confidence when 
mixing formulas calling for either metabisulphite or bisul- 
phite. 


POTASSIUM METABISULPHITE is often used as a preservative. It 
forms good crystals and is convenient to use but is very costly in comparison 
with sodium bisulphite. 


SODIUM BISULPHITE, when pure, is a white salt which has an acid 
reaction, often containing a slight excess of sulphur dioxide. Since sodium 
sulphite is an alkaline salt, owing to the predominance of the strong base, 
soda, over the weak sulphurous acid, a neutral solution can be produced by 
adding a small quantity of bisulphite to sulphite, and this neutral solution 
has found extensive application as a preservative for a pyro developer. Bi- 
sulphite is used very largely as a preservative for fixing baths, supplying 
both the sulphite and the acid necessary. 


It is difficult to prepare bisulphite free from iron, and any iron in the 
bisulphite produces a dark color when used for making up a pyro solution. 


It is often customary to substitute sodium bisulphite for 
potassium metabisulphite weight for weight. It really sim- 
mers down to a matter of dollars and cents because either 
chemical is quite satisfactory for the purpose but, as a rule, 
sodium bisulphite ranges in cost from one-third to one-half 
that of potassium metabisulphite. 


Since sodium bisulphite may be considered as a compound 
of sodium sulphite and sulphurous acid, while sodium sulphite 
is alkaline, bisulphite is preferable as a preservative in the case 
of a two-solution developer, since oxidation progresses less 
readily in acid than in alkaline solution. 


In the case of a one-solution developer containing, say, 
sodium sulphite, sodium bisulphite and sodium carbonate, the 
bisulphite 1s converted to sulphite by the aod carbonate - 
according to the following equation: 


NaHSO3 a Naz2CO3 = Na2SO3 + NaHCOs3 
Sodium Bisulphite Sodium Carbonate Sodium Sulphite Sodium Bicarbonate 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 27 


The sodium bisulphite neutralizes or destroys an equiva- 
lent quantity of sodium carbonate, thus reducing the pro- 
portion of alkali and therefore exerts an apparent restraining 
action, while the developer apparently keeps longer because 
some of the carbonate has been destroyed. 


It might be thought from the above equation that it 
would be as effective and perhaps simpler to use only sodium 
sulphite instead of sulphite and bisulphite, but experiments 

have shown that the bicarbonate formed, acts as an anti- 
fogging agent. 

A further discussion of the properties of developers is 
given under Chapter X, page 83. 


28 EASTMAN KODAK COMPANY 


CHAPTER IV. 


The Chemistry of Fixation 


After development, the undeveloped silver bromide is re- 
moved by immersion of the negative or print in what is called 
the “fixing” bath. There are only a few substances which will 
dissolve silver bromide, and the one which is universally used 
in modern photography is sodium thiosulphate, NazS.Os, 
which is known to photographers as hyposulphite of soda, or 
more usually as hypo, though the name hyposulphite of soda 
is used by chemists for another substance. 


SODIUM THIOSULPHATE or HYPO can be made by boiling together 
sodium sulphite and sulphur, the sulphur combining with the sodium sulphite 
according to the equation. 


Na2SO3 of S = Na2S203 
Sodium Sulphite Sulphur Hypo 


In practice it is generally made from calcium sulphite residues, the calcium 
thiosulphate being then converted into the sodium salt by treatment with 
sodium sulphate. The hypo comes on the market in clear crystals and is 
usually fairly pure, any foreign substance present being more often due to 
accidental contamination than to its chemical nature and consisting of dirt, 
straw or wood dust due to careless handling. Sometimes, however, the hypo 
contains calcium thiosulphate, which decomposes much more readily than the 
sodium salt. On the whole, it is not difficult to obtain good hypo; the East- 
man Tested Hypo is prepared in the form of granular crystals, easy to dis- 
solve, and free from accidental contamination. 


Fixing is accomplished by means of hypo only, but mate- 
rials are usually transferred from the developer to the fixing 
bath with very little rinsing so that a good deal of developer 
is carried over into the fixing bath, and this soon oxidizes in 
the bath, turning it brown, and staining negatives or prints. 
In order to avoid this the bath has sodium sulphite added to 
it as a preservative against oxidation, and the preservative 
action is, of course, greater if the bath is kept in a slightly 
acid state. In order to prevent the gelatin from swelling and 
softening it is also usual to add some hardening agent to the 
fixing bath so that a fixing bath instead of containing only 
hypo will contain in addition sulphite, acid, and hardener. 


If a few drops of acid, such as sulphuric or hydrochloric 
acid are added to a weak solution of hypo, the hypo will be 


EEEMENTARY PHOTOGRAPHIC CHEMISTRY — 29 


decomposed and the solution will become milky, owing to 
the precipitation of sulphur. This is because the acid con- 
verts the sodium thiosulphate into the free thiosulphuric 
acid, and this substance is quite unstable, decomposing into 
sulphurous acid and sulphur according to ‘the equation: 


H2S203 — H2SO3 + S 
Thiosulphuric Acid §Sulphurous Acid = Sulphur 


The change of thiosulphate into sulphite and sulphur is rever- 
sible, since, if we boil together sulphite and sulphur we shall 
get thiosulphate formed, so that while acids liberate sulphur 
from the hypo, sulphite combines with the sulphur to form 
hypo again. Consequently, we can prevent acid decomposing 
the hypo if we have enough sulphite present, since the sul- 
phite works in the opposite direction to the acid. An acid 
fixing bath, therefore, is preserved from decomposition by the 
sulphite, which also serves to prevent the oxidation of de- 
veloper carried over into it. The developer which is carried 
over into the fixing bath is, however, alkaline and conse- 
quently a considerable quantity of acid is required in a fixing 
bath which is used for any length of time, since if only a 
small quantity is present, it will soon be neutralized by the 
developer carried over. We are, therefore, in the difficult 
position that we require a large quantity of acid present, and 
yet the fixing bath must not be strongly acid. The solution 
of the difficulty is found by taking advantage of the fact that 
there are some acids which are very weak in their acidity and 
yet can neutralize alkali in the same way as a strong acid, 
so that a large quantity of these acids can be added without 
making the bath so acid that sulphur is precipitated. 


The strength of an acid depends upon the fact that when 
it is dissolved in water some of the hydrogen contained in it 
dissociates from the acid and remains in the solution in an 
active form, and the acidity of the solution depends upon the 
proportion of the hydrogen which is dissociated into this 
active form. The quantity of alkali which the acid can neu- 
tralize, however, depends upon the total quantity of the hy- 
drogen present, and not on the dissociated portions only. The 
strongest acids are the mineral acids, such as sulphuric and 
hydrochloric while the weakest acids are the organic acids, 
such as citric and acetic acids. 


Since a large quantity of a weak acid is required, the best 
acid for the purpose is acetic acid. 


30 EASTMAN KODAK COMPANY 


ACETIC ACID is prepared by the fermentation of apple juice, yielding 
a product commonly called vinegar. In addition to acetic acid, vinegar 
also contains many impurities and the acid strength is from 4% to 8%. The 
stronger acid is made from acetate of lime which is prepared either by neu- 
tralizing vinegar with chalk or, more commonly, by neutralizing with lime 
the crude acetic acid prepared by the destructive distillation of wood. Acid 
thus prepared may contain as high as 99.5% acetic acid and is usually called 
glacial acetic acid because, at moderately low temperatures, it freezes to a 
solid. Dilutions of the glacial acid are commonly supplied containing 80% 
and 28% acetic acid. This 28% acetic acid, prepared by diluting three parts 
of pure glacial acetic acid with eight parts of water, must not be confused 
with “commercial 28% acetic acid” which is prepared by redistilling the 
acid obtained by the destructive distillation of wood. 


When acetic acid cannot be obtained for the fixing bath, 
the only substitute which appears to be generally available is 
sodium bisulphite, NaHSO;. This compound is intermediate 
between sodium sulphite and sulphurous acid, and is, there- 
fore, equal in acidity to a mixture of equal proportions of 
these two substances. It makes a satisfactory acid fixing 
bath but does not give quite as good a reserve of available 
acid in the bath as acetic acid does. This is of importance 
particularly in connection with the hardening agent used in 
the fixing bath. 


The commonest hardening agent is potassium alum, the 
alums having the property of tanning gelatin. 


ALUMS are compound sulphates consisting of sodium, potassium or 
ammonium sulphate with aluminum sulphate. They have the general 
formula R2SO4, M2(SO4) 3. 24H2O, where R may be an alkali metal or the 
ammonium radical, and M is chromium or aluminum. If the hydrogen 
in sulphuric acid be replaced by potassium, we get potassium sulphate, 
K2SOx4, while if it be replaced by aluminum, we get aluminum sulphate, 
Alg (SO4) 3. The aluminum sulphate combines with other sulphates to form 
the alums, of which the commonest are potassium alum and ammonium 
alum. Sodium alum does not crystallize well, but the potassium and am- 
monium salts crystallize in large, clear crystals. They are almost always 
sold in the form of very fine crystals, to facilitate weighing and to prevent 
lumping which occurs with powdered alums. 


POTASSIUM CHROME ALUM, K,SOx4, Cro(SO4)s3. 24H,O, which 
is often used in the place of ordinary alum, does not contain any aluminum 
in spite of its name. It is a compound sulphate of potassium sulphate with 
chromium sulphate, of which the formula is Cr2(SO4)3, the chromium taking 
the place of the aluminum present in aluminum sulphate. Chrome alum is 
prepared commercially in large quantities and of a high degree of purity. It 
occurs as violet crystals soluble in water, its solution in cold water being 
violet but going green on heating owing to the change in the composition 
of the salt. 


AMMONIUM CHROME ALUM (NH4)2SO«, Cr2(SOu)3 . 24H20, is 
chemically very similar to potassium chrome alum except that it is the 
compound of ammonium sulphate with chromium sulphate. Despite its very 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY _ 31 


close chemical similarity to the potassium compound if a fixing bath con- 
taining ammonium chrome alum becomes alkaline ammonia gas is 
evolved which produces a greenish sheen on the surface of the emulsion 
known as “dichroic” fog. This warning is given because at times, due to 
price fluctuations, the use of ammonium alum is sometimes slightly ad- 
vantageous from the point of view of dollars and cents, but it is always safer 
to use a potassium salt even though it may cost one or two cents a pound 
more. 


In the presence of sodium sulphite a solution of chrome 
alum loses its hardening properties somewhat rapidly, de- 
pending upon the concentration of the chrome alum and the 
sodium sulphite. A fresh chrome alum fixing bath contain- 
ing hypo, chrome alum, sulphite and sulphuric acid loses 
its hardening properties in the course of one or two days even 
if the bath is not used. A chrome alum fixing bath contain- 
ing from I to 2% chrome alum is only useful when used im- 
mediately after preparation, although a bath containing from 
5% to 10% chrome alum will maintain its hardening prop- 
erties for two or three days. Chrome alum is most useful as 
a hardening bath between developing and fixing. A plain 
solution of chrome alum retains its hardening properties 
indefinitely, though with use when developer is carried over 
by the plates and films, the hardening properties of the bath 
fall off owing to the presence of sodium sulphite in the de- 
veloper. 

Formalin is sometimes suggested as a hardening agent in 
fixing baths for hot weather processing. It should be used 
only in alkaline or neutral solutions because in acid solutions, 
it does not harden. With certain individuals, formalin irri- 
tates the mucous membranes of the nose and throat and its 
use, therefore, is very objectionable. 


FORMALIN is a solution of formaldehyde, a gas having a very strong 
odor. The commercial solution contains 40% of formaldehyde and has the 
property of hardening gelatin very powerfully, a 5% solution rendering 
the gelatin of a film completely insoluble in boiling water in less than a 
minute, 


| The characteristics of fixing baths are discussed further 
under Chapter X, page 89. 


32 EASTMAN KODAK COMPANY 


CHAPTER V. 


The Chemistry of Washing 


It'may seem strange that a chapter dealing with washing 
should be inserted in a book on photographic chemistry, be- 
cause washing 1s not usually regarded as a chemical operation. 
Nevertheless, the laws governing washing are distinctly 
chemical in their nature, and the importance of washing in 
photography justifies greater attention than is usually paid 
to the subject. 


As a general rule the object in washing negatives or prints 
is to remove from them the chemicals of the fixing bath which 
they contain. In the first place, it must be pointed out that it 
should not be necessary to wash out silver compounds but 
only the chemicals of the fixing bath. If an exhausted fixing 
bath is used silver compounds will be present during washing 
and must be removed very completely, so that if work has 
to be hurried and the time of washing must be cut down, it 
is most important that fixing should be complete. 


The best way of insuring complete fixing is to use two 
fixing baths, and to transfer the negatives or prints to the sec- 
ond bath after they have been fixed in the first. Then, when 
the first bath begins to show signs of exhaustion and refuses 
to fix quickly, it should be replaced by the second, and the 
new, clean fixing bath should be used in the place of the 
second bath again. 


The rate of washing depends largely upon the rate of 
diffusion of the hypo out of the film into the water providing 
the water in contact with the film is continuously removed. 
This diffusion rate has nothing to do with solubility. The 
solubility of a substance fixes the proportion of the substance 
which can go into solution. 


There are a number of errors which are current concerning 
washing. It is commonly believed, for instance, that plates 
and paper can be washed more rapidly in warm water than in 
cold. This is a mistake. It is true that any salt will diffuse 
more rapidly in warm water than in cold, but when washing 
a photographic material the diffusion has to take place in 
gelatin, and the warmer the water in which the gelatin is 
placed, the more it swells, and its swelling hinders diffusion 


~~ 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 33 


in about the same proportion as the rise in temperature 
accelerates it, so that, as a matter of fact, washing goes on 
at about the same rate at all ordinary temperatures. 


It is sometimes stated that material which has been hard- 
ened in the fixing bath washes more slowly than material 
which has not been hardened. This, too, is incorrect. Gela- 
tin is like a sponge; the effect of hardening it is to contract all 
the network of the sponge, but in so doing the gelatin as a 
whole is not contracted and there is no difference in the diffu- 
sion between gelatin, which has not been hardened and which 
has been hardened, unless the gelatin has been dried after 
hardening. Ifa negative is thoroughly hardened in the fixing 
bath and then is dried down, it will not expand much when 
soaked again and consequently diffusion through it will be 
difficult, but before drying the hardening does not affect dif- 
fusion and the materials which wash most quickly are those 
in which the gelatin has not been swollen in its treatment, 
either in development or fixation, but has been kept in a firm, 
solid condition. 

The actual rate of washing may be understood by remem- 
bering that the quantity of hypo remaining in the gelatin is 
continually halved in the same period of time as the washing 
proceeds. An average negative, for instance, will give up half 
its hypo in fifteen seconds when washed directly under the 
faucet, so that at the end of fifteen seconds half the hypo 
will be remaining in it, after thirty seconds one-quarter, after 
forty-five seconds one-eighth, after one minute one-sixteenth, 
and soon. It will be seen that in a short time the quantity of 
hypo remaining will be infinitesimal. This, however, assumes 
that the negative is continually exposed to fresh water, which 
is the most important matter in arranging the washing of 
either negatives or prints. 

In most trays and washing tanks an average negative will 
give up half its hypo in 30 seconds. The process will then 
stop unless the water in the vessel is changed. The rate of 
washing thus turns out to be dependent firstly, on the degree 
of agitation, and secondly, on the rate of removal of the used 
water. This rate is dependent directly on the ratio of the 
_ stream of water falling into the vessel and the size of the 
vessel, quickest renewal taking place when the vessel is small 
and the stream large. 

Only exact experimental measurements will tell when a 
particular vessel and stream of water will wash a specific 


kind of film or paper. 


34 EASTMAN KODAK COMPANY 


However, as a rough and safe working guide, the washing 
power of the vessel may be judged by noting the time it takes 
colored water to be replaced by colorless water from the 
faucet. To this time the minimum washing time for the ma- 
terial is added, and the total taken to indicate the actual 
period to be allowed by the photographer under his particular 
conditions. 


A practical example will make the matter clear. Suppose 
that a large tray is resting in the sink and water is falling into 
it from the faucet and flowing to waste over the sides. Into 
the tray, and while the water is running, an ounce of 1% 
potassium permanganate solution or red ink is poured in and 
the time noted for the water to become completely colorless. 
To this time is added the minimum washing time of the ma- 
terial, a representative list of which is given below: 


Lantern slide plates: 9. 2 a 3 minutes 
Other plates fs) uf -./2 a 5 minutes 
Film negatives, allkinds  . . . . ¢ \. 90)) ee OURee eee 
Single weight Velox . 3... >...) 
Single weight Bromide . 2. . . . %, | os) ei eee 
Double weight Bromide. . . . . + . ()8y 9) 3G to Gomes 


If it be found that the water in the tray takes Io minutes 
to clear, then the time for a lantern plate to become thor- 
oughly washed would be 13 minutes and for Velox about 25 
minutes. If the water supply were doubled and the rate of 
color discharge shortened to 5 minutes the washing times 
would be 8 minutes and 20 minutes respectively. Finally, if 
the lantern plate were held under an open faucet in the hand, 
the renewal of water at the emulsion surface would be ex- 
tremely rapid and the plate would be safely washed in 3 
minutes. ‘The moral is to use plenty of water and plenty of 
agitation. 

If a lot of prints are put in a tray and water allowed to 
splash on the top of the prints, it is very easy for the water on 
the top to run off again, and for the prints at the bottom to 
lie soaking in a pool of fairly strong hypo solution, which is 
much heavier than water and which will fall to the bottom 
of the tray. If the quickest washing is desired, washing 
tanks should be arranged so that the water is changed con- 
tinuously and completely and the prints or negatives are 
subjected to a continuous current of fresh water. If water 
is of value, and it is desired to economize in its use, then by 
far the most effective way of washing is to use successive 


PaeeranyY PHOTOGRAPHIC CHEMISTRY 35 


changes of small volumes of water, putting the prints first 
in one tray for two to five minutes, and then transferring 
them to an entirely fresh lot of water, and repeating this pro- 
cedure about six times. 


Now a word of warning about contaminating partly 
washed photographs. If the hypo comes out to half its pre- 
vious strength in a fraction of a minute, so also will the re- 
verse change occur. If a partly washed print 1s touched by 
fingers contaminated with the fixing bath, in a few seconds 
the hypo will be shared between the fingers and the photo- 
graph, and it will take the full washing time to remove it 
again. 

Since hypo is invisible and its evil effects are not detected 
till long afterwards, prints or films fresh from the hypo should 
never be placed among those partly washed. If this is done 
the entire batch must be washed from the time that the 
last one was added. 


The best way to avoid contamination is to wash in cascade. 
In its simplest form this can be accomplished by placing two 
trays side by side, one an inch or so above the level of the 
other. Water is allowed to run into the upper tray and over- 
flow into the lower. All prints and films should be placed in 
the lower tray before transference to the upper. The operator 
should also use the lower tray for washing hypo off his fingers. 
The prints and films should have at least three minutes pre- 
liminary washing before going into the true washing tray. 
With a little experience in handling materials in this serial 
order the photographer will be surprised at the number of 
films and papers that can be passed through quite moderate 
capacity trays and completely satisfactory washing secured 
in a short time. 


The progress of washing can be followed by removing two 
or three prints at intervals from the bath and testing for 
hypo by using the hypo test formula, directions for which 
are given on page 57. An even simpler test is to taste the 
prints since hypo containing silver has a sweet taste. 

Six changes of water, allowing five minutes for each 
change, should be sufficient to eliminate the hypo effectively 
from any ordinary photographic material. 


36 EASTMAN KODAK COMPANY 


CHAPTER VI. 


The Chemistry of Reduction and 
Intensification 


Reduction 


By reduction in photography is meant the removal of some 
silver from the image so as to produce a less intense image. 
Thus, in the case of an over-developed film or plate there will 
be too much density and contrast, and the negative may be 
reduced to lessen this. In the case of an over-exposed nega- 
tive there may not be an excess of contrast but the negative 
will be too dense all over, and in this case what is required — 
is the removal of the excess density. 


It is unfortunate that the word “reduction” is used in 
English for this process. In other languages the word 
““weakening”’ is used, and this is undoubtedly a better word, 
because the chemical action involved in the removal of silver 
from a negative is oxidation, and the use of the word reduc- 
tion leads to confusion with true chemical reduction, such as 
occurs in development. 


All the photographic reducers are oxidizing agents, and 
almost any strong oxidizing agent will act as a photographic 
reducer and will remove silver, but various oxidizing agents 
behave differently in respect to the highlights and shadows of 
as image. Reducing solutions can be classified in three 
classes: , 


A. Cutting reducers 
B. True scale reducers 
C. Flattening reducers. 


A. Tue Cuttinc REpucEeRS remove an equal quantity 
of silver from all parts of the image, and consequently remove 
a larger proportion of the image from the shadows than from 
the highlights of the negative. The typical cutting reducer 
is that known as Farmer’s Reducer, consisting of a mixture of 
potassium ferricyanide and hypo, the potassium ferricyanide 
oxidizing the silver to silver ferrocyanide and the hypo dis- 
solving the latter compound. Farmer’s Reducer will not 
keep when mixed, decomposing rapidly, so that it is usually — 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY — 37 


prepared by making a strong solution of the ferricyanide and 
then adding a few drops of this to a hypo solution when the 
reducer is required. It is especially useful for clearing nega- 
tives or lantern slides which show slight fog, and is also used 
for local reduction, the solution being applied with a brush 
or a wad of absorbent cotton. (See formula R-4, p. 58.) 


Another cutting reducer is permanganate. The perman- 
ganates are very strong oxidizing agents, and if a solution of 
permanganate containing sulphuric acid is applied to a nega- 
tive, it will oxidize the silver to silver sulphate, which is 
sufficiently soluble in water to be dissolved. (See formula 
I-28, 0, 58.) 

Permanganate has only a very weak action on a negative 
if no acid is present and this may be made use of for the 
removal of “‘dichroic”’ fog, the yellow or pink stain sometimes 
produced in development. Dichroic fog consists of very finely 
divided silver and this is attacked. by a solution of plain per- 
manganate (about 0.25%) which will have no appreciable 
action on the silver of the image. 


An important difference should be noted between the be- 
havior of ferricyanide and permanganate when used for 
reducing pyro-developed negatives. In a negative developed 
with pyro the image consists partly of the oxidation product 
of the pyro associated with the silver. (See p. 25). When 
such a negative is reduced with ferricyanide the silver is 
removed but the stain is unattacked so that the negative 
appears to become yellower during reduction, though the 
ferricyanide does not really produce the color, only making 
it evident by removal of the silver. Permanganate, on the 
other hand, attacks the stain image in preference to the silver 
and consequently makes the negative less yellow. Perman- 
ganate can also be used as an alternative to ferricyanide for 
bleaching negatives, since if bromide be added to the solu- 
tion, silver bromide will be formed and the same bleaching 
action obtained as with ferricyanide. 


POTASSIUM PERMANGANATE occurs in dark purple crystals 
which dissolve to form a purple solution. It is easily obtained pure but 
there is a good deal of impure permanganate on the market; Eastman 
Tested Permanganate is a pure product. 


In addition to its use for reduction and bleaching, per- 
manganate is employed as a test for hypo, since it is at once 
reduced by hypo, and the colored solution of the permanga- 
nate, therefore, loses its color in the presence of any hypo. 


38 EASTMAN KODAK COMPANY 


It may consequently be used to test the thoroughness of the 
elimination of hypo from negatives or prints in washing. 
When permanganate is reduced in the absence of an excess of 
free acid, a brownish precipitate of manganese dioxide is ob- 
tained and sometimes negatives or prints which have been 
treated with permanganate are stained brown by this ma- 
terial. Fortunately, manganese dioxide is removed by bi- 
sulphite, which reduces it still further, forming a soluble 
manganese salt. The brown stain can, therefore, be removed 
by immersion of the stained material in a solution of bi- 
sulphite. 

A very powerful cutting reducer is made from a solution 
of iodine in potassium iodide, to which potassium cyanide has 
been added to dissolve the silver iodide formed during reduc- 
tion. Iodine is not soluble in water but is soluble in a solution 
of potassium iodide. To make up the reducer a few iodine 
crystals are dissolved in a 10% solution of potassium iodide, 
and five parts of this are added to one part of a 10% solution 
of potassium cyanide, making up to Ioo parts with water for 
use. 

B. Proporrionat Repucers are those which act on all 
parts of the negative in proportion to the quantity of silver 
present there; hence they exactly undo the action of develop-. 
ment, since during development the density of all parts of 
the negative increases proportionally. A correctly exposed 
but over-developed negative should be reduced with a pro- 
portional reducer. Unfortunately, there are no single sub- 
stances which form exactly proportional reducers, but by 
mixing permanganate, which is a slightly cutting reducer, 
with persulphate, which is a flattening reducer, a proportional 
reducer may be obtained. (See formula R-5, p. 58.) 


C. FLATTENING Repucers are those which act very much ~ 
more on the heavy deposits than on the light deposits of the 
negative, and which will consequently reduce the highlights 
without affecting the detail in the shadows. Only one such 
reducer is known, and this is ammonium persulphate. Am- 
monium persulphate 1s a powerful oxidizing agent and attacks 
the silver of the negative, transforming it into silver sulphate, 
which dissolves in the solution. It must be used in an acid 
solution and is somewhat uncertain in its behavior, occasion- 
ally refusing to act, and always acting more rapidly as the 
reduction progresses. (See formula R-1, p. 57.) 


AMMONIUM PERSULPHATE is a white crystalline salt, stable when 
dry. It has been found in the Research Laboratories of the Eastman Kodak 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY = 39 


Company that the action of persulphate depends largely upon its contain- 
ing a very small quantity of iron salt as an impurity, and that its capricious 
behavior is due to variations in the quantity of iron present. The persul- 
phate supplied as an Eastman Tested Chemical may be relied upon to give 
a uniform action in reduction. 


Intensification 


Intensification is photographically the opposite of reduc- 
tion, the object being to increase contrast. This is done by the 
deposition of some material on the silver image. A silver 
image, for instance, can be very much intensified by toning it 
with uranium (see page 44), the reddish-brown uranium 
ferrocyanide having very great printing strength and convert- 
ing a weak negative into one having a great effective contrast 
for printing purposes. Usually, however, intensification is 
performed by depositing a silver, mercury or a chromium 
compound upon the image, and many photographic intensi- 
fiers depend upon the use of mercury. But experience has 
shown that mercury intensified images are not as stable as 
images produced by chromium intensification. 


Mercury is a metal which forms two series of salts, the 
mercuric salts, which are in a higher degree of oxidation, and 
the mercurous salts. 


Many of the mercuric salts are insoluble in water, but mer- 
curic chloride is sufficiently soluble for practical use, and when 
a silver image is placed in a solution of mercuric chloride, this 
reacts with the silver and forms a mixture of mercurous chlo- 
ride and silver chloride. 


The bleached image, which appears white, can then be 
treated in various ways. If it is developed, for instance, both 
the silver chloride and the mercurous chloride will be reduced 
to the metal, and in addition to the silver, with which we 
started, we shall have added to every part of silver an equal 
part of mercury. Instead of using a developer we may blacken 
the image with ammonia, which forms a black mercury am- 
monium chloride and produces a high degree of intensifica- 
tion. 


MERCURY BICHLORIDE or Mercuric Chloride is a virulently poison- 
ous salt, sometimes known popularly as “corrosive sublimate.” Its only 
use in photography is for intensification, and it is obtained in white, heavy 
crystals which are soluble with some difficulty in water. This may be ob- 
tained in satisfactory purity by ordering Eastman Tested Mercury Bichlo- 
ride. 


For many purposes separate bleaching and redevelopment 
is inconvenient, and for this reason the Eastman Intensifier 


40 EASTMAN KODAK COMPANY 


has been placed on the market, with which the intensification 
proceeds continuously so that it can be stopped at any time. 
This does not give quite so great an intensification as the use 
of the two solutions, but it is far more convenient in opera- 
tion. 


A very powerful method of intensification, used chiefly for 
negatives made by photo-engravers, is obtained by bleaching 
with mercuric chloride and blackening with silver dissolved in 
potassium cyanide. The use of the cyanide cuts the shadows 
very slightly at the same time that the highlights are intensi- 
fied, so that a great increase in the contrast of the negative is 
obtained. This is usually known as the ““Monckhoven”’ Inten- 
Sifter. 

In the case of the chromium intensifier the silver image is 
bleached with a solution of bichromate containing a very little 
hydrochloric acid, bichromate being an oxidizer of the same 
type as permanganate or ferricyanide. The image is then 
redeveloped and will be found to be intensified to an appre- 
ciable extent. This intensifier has found increasing favor 
owing to the ease and certainty of its operation and the per- 
manency of the intensified image. 


POTASSIUM BICHROMATE is made by the oxidation of chromium 
salts. It forms orange-red crystals, stable in air, and easily dissolves, yield- 
ing a yellow solution. It is obtained in a pure form by crystallization. Po- 
tassium bichromate is used in photography both for bleaching negatives and 
for sensitizing gelatin, fish glue, etc. When gelatin containing bichromate 
is exposed to light it becomes insoluble in water and in this way images may 
be obtained in insoluble gelatin. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 41 


SEareeR Vit; 


The Chemistry of Toning 


The operation of toning consists in the deposition.on the 
silver image of another substance having a different color, in 
order to get a more pleasing result, or of the transformation 
of the silver image into another substance for the same pur- 
pose. 


There are four principal methods of toning: 


A. Toning by the replacement of the silver by other 
metals; 


B. Toning by the deposition of salts of metals; 

C. Toning by the transformation of the silver image into 
some substance to which dyes will attach them- 
selves in an insoluble form; 

D. Transformation of the ives image into a stable, 
strongly colored salt of silver. 


A. Inthe case of prints which are made by the printing- 
out processes, the silver compound produced by the action of 
light is colored, and after fixation the image left is usually of 
an unpleasant color, a yellow or yellow-brown. In order to 
change this to a more satisfactory color it is toned by means 
of gold or, more rarely, platinum. 


When a finely divided silver image is placed in a solution 
of gold or platinum the silver will replace the metal in solu- 
tion, going into solution itself, and the gold or platinum will 
be deposited in the place of the silver. The rate at which 
these metals are deposited is very important, especially in 
the case of gold toning. If the gold is deposited too slowly, 
it will be deposited in a very fine condition, and in the case 
of finely divided metals, their color depends upon the fineness 
of the division. Finely divided gold is red, which is not as 
pleasing as the blue gold obtained by more rapid deposition. 


To insure rapid deposition it is necessary that the bath 
should be kept alkaline, and consequently borax or sodium 
acetate is added to the gold chloride to make a toning bath, 
while sometimes substances having a weak reducing action 
are added, such as sulphocyanides or formates. Platinum 
toning baths are used in an acid condition. 


42 EASTMAN KODAK COMPANY 


The chemicals used for making up these toning baths 
must be of high purity, and it is best to get tested chemicals 
in all cases. 


GOLD CHLORIDE is made by dissolving gold in a mixture of hydro- 
chloric and nitric acids and evaporating the solution. It forms brownish 
crystals, rapidly absorbing water, which contains 65% metallic gold. The 
salt is sold in small glass tubes containing 15 grains, and in order to use it, 
the label is removed from the tube and the tube is broken in a bottle con- 
taining a known volume of water so that a solution of definite strength is ob- 
tained without danger of losing the precious material. 


GOLD SODIUM CHLORIDE is a double chloride of gold and sodium 
which occurs in yellow crystals and contains 49% of metallic gold. It has 
the advantage over the pure chloride of gold that it is neither acid nor deli- 
quescent. 


POTASSIUM CHLOROPLATINITE is the double chloride of platinum 
and potassium, and is the form in which platinum is used for a toning bath. 


It occurs in reddish crystals, and is supplied in sealed glass tubes like gold 
chloride. 


LEAD NITRATE and LEAD ACETATE. These colorless salts of lead 
are sometimes used for toning baths. They are both soluble in water and 
the solutions are very poisonous. 


SODIUM ACETATE, SODIUM PHOSPHATE (Di-basic) and BORAX 
are all weak alkalis and are used in gold toning baths for this reason. They 
occur as white salts, soluble in water. Borax occurs as a mineral and is 
largely used in industry. Only the pure salt should be used for photographic 
purposes. 


AMMONIUM SULPHOCYANATE, SULPHOCYANIDE OR THIO- 
CYANATE, is a salt occurring in very deliquescent crystals. In order to be 
at all certain of its strength it must be preserved with great care, out of con- 
tact with the air. It is one of the most popular salts for use with gold chloride 
in toning baths. 


B. Agood many metallic compounds are colored, and if 
the silver image is replaced by these colored compounds, 
wholly or in part, a colored image is obtained. In most of the 
toning processes based upon the use of colored compounds, 
ferrocyanides of metals are employed, the silver image being 
first transformed into silver ferrocyanide, the silver in the 
silver ferrocyanide being then substituted by another metal 
of which the ferrocyanide is colored. 


The ferro- and ferricyanides are very complex compounds. 
The cyanides themselves are compounds containing carbon 
and nitrogen, and have a curious resemblance to chlorides and — 
bromides. Hydrogen unites with carbon and nitrogen to form 
an acid, HCN, which is called Aydrocyanic acid, or some- 
times prussic acid. The hydrogen in this can be substituted 
by metals to form cyanides such as potassium cyanide, KCN, 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY — 43 


which is analogous to potassium chloride, KCl, or potassium 
bromide, KBr, and on adding a solution of silver nitrate to 
a soluble cyanide, silver cyanide, AgCN, is precipitated as 
an insoluble salt, just as silver chloride or silver bromide is 
precipitated. 


There is one respect, however, in which hydrocyanic acid 
and the cyanides differ from the corresponding chlorine or 
bromine compounds, and this is that they are extremely poi- 
sonous. 4 few grains of cyanide swallowed will cause death. 


Cyanide solutions are solvents for the silver halides, form- 
ing soluble double compounds with the insoluble silver salts. 
Potassium cyanide is employed for fixing wet collodion plates, 
which, being made from silver iodide, are not easily fixed in 
hypo. Whenever cyanides are used by photographers, their 
extremely poisonous nature should be remembered and every 
possible precaution taken in keeping and using them. 


The cyanides easily form complicated double compounds. 
With sulphur, for instance, they form sulphocyanides, and 
ammonium sulphocyanide has already been referred to as 
being used in gold toning baths. The cyanides unite with iron 
cyanides to form two important groups of compounds called 
ferrocyanides and ferricyanides. hese differ from each other 
in their degree of oxidation, the ferricyanides being more 
highly oxidized than the ferrocyanides, so that when a ferri- 
cyanide is reduced a ferrocyanide is formed. 


POTASSIUM FERROCYANIDE is yellow. It is infrequently known 
as “‘yellow prussiate of potash” and has very little application in photog- 
raphy. 


POTASSIUM FERRICYANIDE or red prussiate of potash is prepared 
by passing chlorine gas into a solution of the ferrocyanide and is deposited 
from concentrated solution as red crystals. The crystals are soluble in water 
and give a yellow solution. 


The value of ferricyanide in photography lies in the fact 
that ferricyanide oxidizes the silver image and forms silver 
ferrocyanide from it, so that if a negative is placed in a solu- 
tion of ferricyanide, it is slowly bleached to silver ferrocya- 
nide. 


This property can be made use of in various ways. The 
silver ferrocyanide is soluble in hypo so that if we use a solu- 
tion of potassium ferricyanide and hypo instead of plain 
potassium ferricyanide, we shall not get a white image but 
the silver image will be dissolved slowly, since it will be 


44 EASTMAN KODAK COMPANY 


converted into the silver ferrocyanide by the ferricyanide and 
then the silver compound formed will be dissolved in the 
hypo. This mixture of ferricyanide and hypo is known as 
Farmer’s Reducer, and will be referred to in the next chapter. 
Again, if we add bromide to our ferricyanide solution, silver 
bromide is more insoluble than silver ferrocyanide and con- 
sequently the silver ferrocyanide as it is produced will be 
transformed into silver bromide. This operation of trans- 
forming a silver image into a bromide image is generally 
known as bleaching. If we combine with the potassium ferri- 
cyanide a salt of a metal which gives an insoluble colored 
ferrocyanide, then we shall get the silver ferrocyanide formed, 
and this will be converted into the ferrocyanide of the metal 
whose salt has been added to the bath. If we add an iron 
salt, such for instance as iron citrate, to the potassium ferri- 
cyanide, we shall get a blue iron ferrocyanide formed and the 
image will be toned blue. If we use uranium nitrate, we shall 
get the reddish-brown uranium ferrocyanide, while if we use 
copper citrate, we shall get the red copper ferrocyanide. 
Sometimes instead of using the metal salt in the same bath 
as the ferricyanide the operation is done in two steps, the 
silver being first bleached to silver ferrocyanide, which then 
combines with a salt of the metal to form the colored metallic 
ferrocyanide. : 


C. The range of colors which can be obtained by the use 
of colored metals or metallic compounds is rather limited, and 
in order to get a wider range, especially for motion picture 
and lantern slide work, experimenters have tried to find 
methods of using dyes and attaching them to the image. 


It has been found that this can be done by transforming 
the silver image into silver iodide, which can be accomplished, 
for instance, by treatment of the image with a mixture of 
potassium ferricyanide and potassium iodide. The silver 
iodide image formed in this way will mordant basic dyes and 
attach them to the image so that the image assumes the color 
of the dye. The Kodak Research Laboratories have worked 
out a process in which instead of transforming the silver image 
into silver iodide, it is treated with a uranium mordanting 
bath and transformed into a mixture of uranium and silver 
ferrocyanides, and then the basic dyes are mordanted onto 
this image. Full particulars of this process are given in our 
booklet ‘Lantern Slides, How to Make and Color Them,” 
supplied on request to the Service Dept., Eastman Kodak 
Company, Rochester, N. Y. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY = 45 


D. Silver sulphide is a very insoluble compound of 
silver, and consequently if a silver image or a silver halide 
salt is treated with sulphur or a sulphide respectively they 
will at once be transformed into silver sulphide. Silver sul- 
phide has a color varying from light brown to black, accord- 
ing to its state of subdivision, and the transformation of the 
image into silver sulphide is by far the most popular method 
of toning developing-out paper prints, the prints so toned 
being generally known as “sepia’’ prints. 

When great permanency is required prints should prefer- 
ably be toned to a silver sulphide image since experience has 
shown that this form of silver is one of the most stable. There 
are two general methods of transforming the image into silver 
sulphide: 

1. Direct toning, with the hypo alum bath; and 
2. Bleaching and redevelopment. 


As was explained in the chapter dealing with fixing, 
Sie an acid is added to a solution of hypo, it tends to precip- 
itate sulphur. Now, a solution of alum in water is weakly 
acid, so that if alum ‘is added to plain hypo without any sul- 
phite present, the solution will, after a time, become turbid 
and precipitate sulphur. This solution of alum and hypo 
at the point where it is ready to precipitate the sulphur may 
be considered as having free sulphur in solution, and if prints 
are immersed in a hot solution of alum and hypo (about 
120° F.) (49° C.), the silver image will be converted directly 
into silver sulphide and the prints will be toned brown. 
Two precautions are necessary in order to obtain successful 
results with the hypo-alum toning bath. The bath works 
best at a temperature of 120°-125° F. (49°-52°C.); if the 
temperature is allowed to rise much above 130° F. (54° C.) 
there is danger of blistering and bleaching of the image. 
A fresh bath tends to weaken the print, eating out the high- 
lights. To prevent this a little silver must be added to the 
bath preferably in the form of silver chloride as given under 
the hypo-alum toning bath formula T-1a, page 60. A bath 
- lasts for a long time, and as a general rule a hypo-alum bath 
which has been somewhat used works better than a fresh 
bath. The color of the final tone is related directly to the 
color of the original black and white image. Blue-black 
images give cold chocolate tones; olive green images give 
warm sepia tones. 

2. It is rather troublesome to use a bath which has to be 
heated, so that while hypo-alum toning is used on the large 


sate EASTMAN KODAK COMPANY 


scale, smaller quantities of prints are commonly toned by 
bleaching the silver print in a bath of ferricyanide and bro- 
mide, and then treating the bleached print after washing, with 
sodium sulphide, which converts the silver bromide directly 
into silver sulphide. This process is quite satisfactory for 
use with amateur prints and enlargements. 


SODIUM SULPHIDE occurs in white, transparent crystals, which have 
a strong affinity for water and quickly deliquesce unless kept carefully pro- 
tected from the air. It is best kept in a strong stock solution. So much 
trouble has been caused by impure sodium sulphide that of recent years the 
Eastman Kodak Company has been supplying a sulphide which they have 
fused so that it will contain no moisture and be of definite purity. One part, 
by weight, of the fused sulphide is equivalent to three parts, by weight, 
(approx.) of the crystals. Fused sodium sulphide is a greyish white product 
but this color is not a sign of harmful impurities. 


Sodium sulphide often contains impurities, chiefly iron, though by dis- 
solving in hot water the iron sulphide separates out as a black sludge, leaving 
a clear solution which should be decanted. Old sodium sulphide often con- 
tains hypo, and if it is present in any considerable quantity, some of the 
silver bromide will be dissolved by the hypo and the print will lose strength 
in the highlights and give an inferior result. 


All sulphides give off a certain quantity of hydrogen sul- 
phide, which smells offensively, and which is extremely dan- 
gerous to unexposed photographic materials, since a very 
small quantity of hydrogen sulphide will convert enough of 
the silver bromide or chloride of the material into sulphide 
to produce a severe fog. No photographic materials should 
therefore be stored in a room where sulphides are kept or 
where sulphide toning is done. 


It has already been explained that the color of silver sul- 
phide depends upon its state of division, and since the state of 
division of the toned image depends upon that of the untoned 
image and this again upon the treatment of the material, it is 
evident that the exposure and development of the print will 
have an effect upon the result obtained. As a general rule, it 
may be stated that to get good colors in sulphide toning it is 
necessary that a print should have been fully developed but 
not over-exposed. If a trace of iron is present in the ferricy- 
anide-bromide solution used as the bleach in the redevelop- 
ment process, for example, from a defective enameled tray, 
blue spots composed of ferric ferrocyanide are liable to form. — 
This tendency to form spots is reduced to a minimum by 
adding potassium oxalate to the bleach bath since the 
blue iron salt is soluble in the oxalate. Acetic acid is added 
also, to prevent possible formation of blisters (see formula 
T-7a, page 60. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY = 47 


CHAPTER VIII. 
Formulas 


It is always best to use the formulas for solutions recom- 
mended in the instructions issued by the maker for the use of 
photographic materials; such formulas are often adjusted to 
the properties of the particular materials concerned and will 
give better and more certain results than can be obtained with 
any others. It is often convenient, however, to have available 
standard formulas, and the following are therefore given: 


Developing Formulas for Films and Plates 


Standard A. B. C. Pyro eee 
Stock Solution A D-1 
; Avoirdupois Metric 
Sodium Bisulphite (E.K.Co.). . . 140 grains 9.8 grams 
Pyro oO ee ae a 2 ounces 60.0 grams 
Potassium Bromide Tee See er kor, sorains 1.1 grams 
Watertomake ..... . . 32 ounces _ 1.0 liter 
Stock Solution B 
WaLetmecmian cis a. s) a sa, va O28. ounces ~ 1,0 liter 
Sodium Sulphite (E. K. Co.) . .. . 314 ounces 105.0 grams 
Stock Solution C 
Witness 5 ~~ 5s. «os o2 ounces. _1.0 liter 
Sodium Carbonate (E. K.Co.) . . 2144 ounces 75.0 grams 


Dissolve chemicals in order given. 
For Tray Development— 

Take 1 part of A, 1 part of B, 1 part of C, and 7 parts of 
water. Develop about 7 to 9 minutes at 65° F. (18° C.). 
For Tank Development— 

Take 1 part of A, 1 part of B, 1 part of C, and 11 parts of 
water. Develop about 13 to 15 minutes at 65° F. (18° C.). 


Two Solution Pyro Tray Developer poe 
Stock Solution A 
Avoirdupois Metric 
Sodium Bisulphite (E. K.Co.) . . 140 grains 9.8 grams 
Pyro PET SM eciy ia Sy eae le 2 ounces 60.0 grams 
Potassium Bromide. . . . . . 16 grains 1.1 grams 
Watertomake .... . . . 32 ounces 1.0 liter 
Stock Solution B 
WKAtCGGEE I, gk eS ae 32 Ounces 1.0:Hter 
Sodium Sulphite (E. K. Co.) . ... 314 ounces 105.0 grams 
Sodium Carbonate (E. K. Co.) ... 214 ounces 75.0 grams 


For use, take 1 part of A, 1 part of B, and 8 parts of water. 
Develop about 6 minutes at 65° F. (18° C.). 


48 EASTMAN KODAK COMPANY 


Tank or Tray Developer — 
(Elon-Pyro) 
Stock Solution A ; 

Avoirdupois Metric 
Water (about 125° F.) (52° C.) . . 16 ounces 500.0 cc. 
Elon Loe ee 4 14 ounce 7.5 grams 
Sodium Bisulphite (E. K. Go. ) i tec 14 ounce 7,5 grams 
Pyro ee Seed 1 ounce 30.0 grams ~- 
Potassium Bromide ace eit A ee tren sane 4.2 grams 
Watertomake .... . . . 32 ounces _ 1.0 liter 


Stock Solution B 


Water. . . «+. 82 oGnces’ f0iliter 
Sodium Suiphite E. ‘K. fern neg eee 5 ounces 150.0 grams 


Stock Solution C - 
Water ... . . . 32 ounces 1.0 liter 


Sodium Gachouate (E. K. Co) hes 21424 ounces 75.0 grams 
For Tray Development— 


Take 1 part of A, 1 part of B, 1 part of C and 8 parts of 
water. Develop about 5 to 7 minutes at 65° F. (18° C.). 


For Tank Development— 


Take 1 part of A, 1 part of B, 1 part of C and 16 parts of 
water. Develop about gi tars minutes at 652 Ey eee 


Tank or Tray Developer | Formula 
(Elon-Hydroquinone) D-6la 
; Stock Solution 
Avoirdupois — Metric 
Water (about 125° F.) (52° C.) Ry 5 16 ounces 500.0 cc. 
2 Elon ae Smee 45 grains 3.1 grams 
\\ Sodium Sulphite . ‘kK. Co. } tetas 3 ounces 90.0 grams 
| Sodium Bisulphite (E.K.Co.) . . 30grains 2.1¢gfams © ~ 
Hydroquinone . a eee 85 grains 5.9 grams ~. 3 ~~: 
Sodium Carbonate (E. K. Co. ) . . 165 grains 11.3 grams 
Potassium Bromide. . .. . . 24 grains 1.7 grams 
Cold water tomake. .... . 32 ounces 1.0 liter 


For tray, use 1 part of stock solution to 1 part of water. 
Develop about 7 minutes at 65° F. (18° C.). 


For tank development of roll films, use 1 part of stock 


solution to 2 parts of water. Develop about 15 minutes at — 
65° RB? -C.). 


For tank development of professional films, use I part of © 


stock solution to 3 parts of water. Develop about 14 minutes 
at 65° F. (18° C.). 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY —§ 49 


Two-Solution Contrast Plate Developer [ Formula 
(Elon-Hydroquinone) D-28 
Stock Solution A 
Avoirdupois Metric 
Water (about 125° F.) (32° C.) - . 16 ounces 500.0 cc. 
Elon : . . . 60 grains 4.2 grams 
Sodium Sulphite ‘E. K. lo: 2 Re), deeeha 114 ounces 45.0 grams 
Hydroquinone.. > . ~. 120 £grains 8.4 grams 
_ Potassium Bromide. . . . . . 50° grains 3.5 grams 
- Watertomake .... . . . 32 ounces _ 1.0 liter 
Stock Solution B 
Water... . . . 32 ounces 1.0 liter 
Sodium Cartonate (E. K. Co. pce er Be 3144 ounces 97.5 grams 


For use, take Stock Solution A, 1 part, Stock Solution B, 
tI part. Develop about 4 minutes at 65° F. (18° C.). 


Process Tray Developer Formula 
(Hydroquinone-Caustic) D-9 
Stock Solution A 
Avoirdupois Metric 
Water (about 125° F.) (52° C.) - . 16 ounces 500.0 cc. 
Sodium Bisulphite (E. K. Co.) .. . 34 ounce 22.5 grams 
Hydroquinone . . et bela 34 ounce 22.5 grams 
Potassium Bromide. . ... . 34 ounce 22.5 grams 
Cold water tomake. . . . . . 32 4ounces 1.0 liter 
Stock Solution B 
Cold water ; . 32 ounces 1.0 liter 
Sodium Hydroxide (Caustic Soda) ; 134 ounces 52.5 grams 


Use equal parts of A and B and develop about three min- 


Mieeeat Ore 1 ( 18° C.). 


Cold water should always be used when dissolving sodium 
hydroxide (caustic soda) because considerable heat 1s evolved. 
If hot water is used, the solution will boil with explosive 
violence and may cause serious burns if the hot alkali spatters 


_on the hands or face. 


Solution A should be stirred thoroughly 


when the caustic alkali is added to it; otherwise the heavy 


caustic solution will sink to the bottom. 


Process Tank or Tray Developer [| Formula 
Elon-Hydroquinone D-11 
Avoirdupois Metric 

Water (about 125°F.) (52°C.) . . . 16 ounces 500.0 cc. 
Elon s . . 14° grains 1.0 gram 
Sodium Sulphite (E. K. Co. as aps 2144 ounces 75.0 grams 
Hydroquinone. . yea ee le LS grains 9.0 grams 
Potassium Carbonate or Pasaion 

Carbonate . ... . . . . 360 grains 25.0 grams 
Potassium Bromide Foe uo cnt, ee ee Ose OTATIIS, 5.0 grams 
Cold water tomake. . . . . . 32 + £4ounces_ 1.0 liter 


50 EASTMAN KODAK COMPANY 


Formula D-11 used at 65° F. (18° C.), in either tank or 
tray will give very good contrast in five minutes. The 
developer is recommended for use with Process and Process 
Panchromatic Films or Plates. 

When less contrast is desired, the developer should be 
diluted with an equal volume of water. 


Tropical Process Developer Formula 
(Kodelon-Hydroquinone) D-13 
Avoirdupois Metric 
Water (about 125° F.) (52° C.) - ». 24 ounces 750.0 cc. 
Kodelon ¢ 75 grains 5.2 grams 
Sodium Sulphite ‘E. K. wer eae 134 ounces 52.5 grams 
Hydroquinone . f . 150 grains 10.5 grams 
Sodium Carbonate (E. K. Ca: ) 134-ounces 52.5 grams 
Potassium Iodide 30 =©grains 2.1 grams 
Sodium Sulphate, Govstats 314 ounces 105.0 grams 
Water to make ieoks 32 ounces 1.0 liter 


Use without dilution. Develop about 6 to 7 minutes at 
85° F., or for proportionately longer times at lower tempera- 
tures. Rinse for 30 seconds and immerse for 3 minutes in a 
5% formalin solution. Then wash for 1 minute, fix in an 
acid hardening fixing bath (Formula F-1) and wash 1§ to 20 
minutes. 


Tray Developer for Roll Film Formula 
(Elon-Hydroquinone- Pyro) D-18a 
Stock Solution 

Avoirdupois Metric 
Water (about 125° F.) 52°C.) . 16 ounces 500.0 cc. 
Elon reser ‘ 20 grains 1.4 grams 
Sodium Sulphite ‘E. me (Cn. Vee 2144 ounces 75.0 grams 
Hydroquinone 80 grains 5.6 grams 
Sodium Bisulphite (E. K. Co. ) 30 8 grains 2.1 grams 
Pyro : %ounce 15.0 grams ™ 
Sodium Gathanase (E. K. Go. ) 1144 ounces 45.0 grams 
Potassium Bromide . : 12 grains 0.8 gram 
Cold water to make . 32 ounces 1.0 liter 


For use, take 1 part of stock solution and 1 part of water. 
Develop 5 to 7 minutes at 65° F. (18° C.). 


Long Life Deep Tank Developer for Roll Film 
(Elon-Hydroquinone-Pyro) 


Formula 
Solution No. 1 D-75 
Avoirdupois Metric 
Water (about 125° F. : phe C.) - 16 ounces 500.0 cc. 
Elon 7 . . 44 grains 3.0 grams 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY _ si 


Solution No. 2 


Avoirdupois Metric 
Water (about 125° F.) (52° C.) . . 16 ounces 500.0 cc. 
Sodium Sulphite (E. K. Co.) . . . 260 grains 18.0 grams 
Sodium Bisulphite (E. K. Co.) ... 114 ounces 36.0 grams 


Solution No. 3 


Hot water (about 160° F.) (71° C.) . 16 ounces 500.0 cc. 

Sodium Sulphite (E. K. Co.) . . . 260 ounces’ 18.0 grams 

Hydroquinone.. .. . . . 175 grains 12.0 grams 

Pyro Mees tie a ce ce, 44°  Srains 3.0 grams 
Solution No. 4 

Water (about 125° F.) (52° C.) . . 16 ounces 500.0 cc. 

Sodium Carbonate (E. K.Co.) ... 2144 ounces 72.0 grams 


Mix each solution separately and add to the tank in the order given. 
Then add watertomake ..... 1 gallon 4.0 liters 


Develop about 15 minutes at 65° F. (18° C.). 


Further details on handling roll films are given in the 
booklet, “Commercial Photo Finishing.” 


X-ray Developer Formula 
(Elon-Hydroquinone) D-19 

Avoirdupois Metric 
Water (about 125° F.) (52° C.) . . 16 ounces 500.0 cc. 
Elon joes + ea aspen oor — STains 2.5 grams 
Sodium Sulphite (E. ae “Co. yas aus Rope 314 ounces 105.0 grams 
Hydroquinone. . . . 140 grains 9.8 grams 
Sodium Carbonate (E. K. Co: ) wens 134 ounces 52.5 grams 
Potassium Bromide. . .. . . 90 grains 6.3 grams 
Cold water tomake. .. . . . 32 ounces 1.0 liter 


Use without dilution. Develop 5 minutes at 65°F. 
pias). 


Motion Picture Developer Formula 
Negative or Positive Film D-16 
Avoirdupois Metric 

Water (about 125° F.) (52° C.) . . 64 ounces 2.0 liters 

Elon A oe eee hee Srains 1.2 grams 
Sodium Sulphite (E. KK: ° Go: VEE ad tama 514 ounces 160.0 grams 
Hydroquinone. . . . . 350 grains 24.0 grams 
Sodium Carbonate (E. K. Co. Spine alae: 214%4 ounces 75.0 grams 
Potassium Bromide Secale laaen Os erains 3.6 grams 
Citric:Acid =... . . . . 40 grains 2.8 grams 
Potassium Metabisulphite wpe coool, .Srains 6.0 grams 
Cold waterto make. .... . 1 gallon 4.0 liters 


Average time of development 7 to 15 minutes at 65° F. 


Prats.) 


52 EASTMAN KODAK COMPANY 


Negative Motion Picture Developer gee || 


(Elon-Hydroquinone) D-71 
Avoirdupois Metric 

Elon hc - « « L715 istems 8.0 grams 
Sodium Sulphite (E. K. Co. De ice ca 2% ounces 75.0 grams 
Hydroquinone . . . 29 grains 2.0 grams 
Sodium Carbonate (E. K. one ) es 134 ounces 50.0 grams 
Potassium Bromide. . . . . . 43 grains 3.0 grams 
Water tomake .. .. STG 2: 1 gallon 4.0 liters 


Time of development 6 to 12 minutes at 65°F. (18° C.). 
Fine Grain Negative Developer Formula 


For Motion Pictures D-76 
(Elon-Hydroquinone-Borax) 

Avoirdupois Metric 
Elon 2 . . 115. grains 8.0 grams 
Sodium Sulphite (E. K. Cn. & . . . 134% ounces 400.0 grams 
Hydroquinone.. . . . . 290 grains 20.0 grams 
Borax . . eo oe i ea See a 8.0 grams 
Water to aie Oe eee Sara 1 gallon 4.0 liters 


Directions for Mixing: ‘Dike the Elon in a small vol- 
ume of water (at about 125° F.) (52° C.) and add the solution 
to the tank. Then dissolve approximately one-quarter of the 
sulphite separately in hot water (at about 160° F.) (71° C.), 
add the hydroquinone with stirring until completely dis- 
solved. Then add this solution to the tank. Now dissolve 
the remainder of the sulphite in hot water (about 160° F.) 
(71° C.), add the borax and when dissolved, pour the entire 
solution into the tank. Dilute to the required volume with 
cold water. 


Time of development is Io to 20 minutes at 65° F. (18° C.). 


Lantern Slide Formulas 


Blue Black Tones Formula 
Stock Solution A D-34 
Avoirdupois Metric 
Water (about 125° F.) (52° C.) . . 16 ounces 500.0 cc. Z 
Elon aay . . « 60% > grains 4.2 grams 
Sodium Sriprare (E. kK. ‘Co! ) Cerra %ounce 15.0 grams 
Hydroquinone ...... . %ounce 15.0 grams 
Cold water to make . .« « + . 32 ounces 1.0 liter 
Stock Solution B 
Water . . - . . 32 ounces 1.0 liter 
Sodium Carbonate (E. K. Ce. yer %ounce 15.0 grams 
Potassium Bromide. . . . . . 30 Bhagehanis) 2.1 grams 


For use, take equal parts of A and B. 
For softer tones, dilute with an equal ae ai: water. 
Develop 1% to 3 minutes at 70° F. (21° C.). 


PrLEMENTARY PHOTOGRAPHIC CHEMISTRY —§ 53 


Warm Black Tones Formula 
Stock Solution A D-32 
Avoirdupois Metric 

Water (about 125° F.) (52° C.) - . 16 ounces 500.0 cc. 
Sodium Sulphite (E. K. Co.) . . . 90 grains 6.3 grams 
Hydroquinone. . . . . . . 100 grains 7.0 grams 
Potassium Bromide. . . . . . 50. grains 3.5 grams 
Citric Acid Me eet Sige Sy OG LO Usrains 0.7 gram 
Cold waterto make. . . . . . 32  4ounces 1.0 liter 

Stock Solution B 
Cold water’ ... . . . 32 ounces 1.0 liter 
Sodium Carbonate (E. K. ee. Neat ier ae 1 ounce 30.0 grams 
Sodium Hydroxide (Caustic Soda) . 60 grains 4.2 grams 


For use, take 1 part of Aandi partof B. For still warmer 
tones, I part of A and 2 parts of B. 


Develop about 5 to 6 minutes at 70° F. (21° C.). 


Developing Formulas for Paper 
Velox, Azo and Bromide Papers he 


Stock Solution D-72 

Avoirdupois Metric 
Water (about 125° F.) (52° C.) . . 16 ounces 500.0 cc. 
Elon rhe ASS Srains 3.1 grams 
Sodium Suiphite (E. ve aa, Duy ak eeaa mete 14%, ounces 45.0 grams 
Hydroquinone . . . . 175 grains 12.2 grams 
Sodium Carbonate (E. K. Co. Nie er amare 214 ounces 67.5 grams 
Potassium Bromide hee a te nee es Orang 1.9 grams 
Cold water tomake. . . . . . 32 + £4ounces 1.0 liter 


For use: Dilute 1 to 1 for Velox; 1 to 2 for Azo and 1 to 
4 for Bromide Papers. 


Develop Velox and Azo 45 seconds at 70° F. (21° C.). 
For colder tones on Azo, dilute as for Velox. 
Develop Bromide papers 1% minutes at 70° F. (21° C.) 


Stock Solution D-51 
Avoirdupois Metric 
Water (about 125° F.) (52° C.) . . 24 ounces 750.0 cc. 
Sodium Sulphite (E. K. Co.) . ... 4 ounces 120.0 grams 
Acrol (Diaminophenol Hydrochloride) 1144 ounces 37.5 grams 
Cold water tomake. . . . . . 32 + ounces 1.0 liter 


For use, take 6 ozs. (180 cc.) Stock Solution, 34 dram 
(4c) 10% potassium bromide solution, and 24 ozs. (750 cc.) 
of water. This developer oxidizes quite rapidly when exposed 


to the air so that only enough developer should be mixed 
for immediate use. 


Acrol Developer for Bromide Papers al 


54 EASTMAN KODAK COMPANY 


Portrait Bromide Paper Developer [ Formula 


Stock Solution D-49 

Avoirdupois Metric 
Water (about 125° F.) (52° C.) - » 16 ounces 500.0 cc. 
Elon 5 . . ». 45 grains 3.1 grams 
Sodium Suiphite (E. .< fo: ) We ee aS 1144 ounces 45.0 grams 
Hydroquinone . . . 165 grains 11.5 grams 
Sodium Carbonate (E. K. Cé: ) vaeete 144 ounces 45.0 grams 
Potassium Bromide. . . . . . 30 £grains 2.1 grams 
Cold water to make. . . . . . 32 ounces 1.0 liter 


For use, take Stock Solution, 1 part, water, 1 part. 
Develop not less than 134 minutes at 70° F. (21° C.). 


Vitava Paper Developer Formula 
Stock Solution — D-52 

Avoirdupois Metric 
Water (about 125° F.) (52° C.) . . 16 ounces 500.0 grams 
Elon ore , i eg. 1225 OS Pana 1.5 grams 
Sodium Sulphite ‘E. Re Go: ) ery h > AP 34 ounce 22.5 grams 
Hydroquinone. . « =» 90) Verains 6.3 grams 
Sodium Carbonate (E. K. Ca: > ee Y%ounce 15.0 grams 
Cold water tomake. . . . . . 32 ounces 1.0 liter 


For use, dilute as follows: 


Vitava Athena—Take Stock Solution, I part, water, I 
part. Add % oz. (8 cc.) of 10% potassium bromide solution 
to each 32 ozs. (1 liter) of developer diluted for use. 


Vitava Alba—Use full strength stock solution. Add 1 dram 
(4 cc.) of 10% potassium bromide solution to each 32 ozs. 
(1 liter) of developer. 7 


Vitava Rapid Black—Use full strength stock solution. Add 
¥% oz. (16 cc.) of 10% potassium bromide solution to each 
32 ozs. (1 liter) of developer. 


Vitava Zelta—Take Stock Solution, 1 part, water, I part. 
Add % oz. (16 cc.) of 10% potassium bromide solution to 


each 32 ozs. (1 liter) of developer diluted for use. 


Develop not less than 114 minutes at 70° F. (21° C.). 


Additional formulas are to be found in “Book of Formulas 
for Eastman Papers,” published by the Eastman Kodak 
Company, Rochester, N. Y 


tte RA ae 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 55 


RINSE BATHS A ae 
Acid Rinse Bath for Paper [ SB-1 | 


After development, rinse prints thoroughly in running 
water or in the following acid rinse bath before placing in the 
fixing bath (Formula F-1). 


Avoirdupois Metric 
BEE HRCLMM Efe he ysl. «+... 02 . Ounces 1.0 liter 
*Acetic Acid (28% pure) (E. K. Co.) . 1144 ounces 48.0 cc. 


*To make 28% acetic acid from glacial acetic acid dilute three parts of glacial 
acid with eight parts of water. 


Chrome Alum Hardening Bath f formuta 
for Films and Plates SB-3 


In hot weather, the following hardening bath should be 
used after development and before fixation in conjunction 
with Formula F-1 or when F-16 does not harden sufficiently. 


Avoirdupois Metric 
Water . refit acalmeerea ie 32 ounces 1.0 liter 
Potassium Chrome Alum... . 1 ounce 30.0 grams 


Agitate the negatives for a few seconds when first im- 
mersed in hardener. This bath should be renewed frequently. 


FIXING BATHS 
Acid Hardening Fixing Bath for sec || 


Films, Plates and Papers F-1 
Avoirdupois Metric 

EV DOMME Cater sc tee eee 16 ounces 480.0 grams 
Water tomake .... ss. . 64 ounces 2.0 liters 


Then add the following hardener solution slowly to the 
cool hypo solution while stirring the latter rapidly. 


Water (about 125° F.) (52° C.) Le 5 ounces 160.0 cc. 

Sodium Sulphite (E. K. Co.) . : 1 ounce 30.0 grams 
*Acetic Acid (28% pure) (E. K. Co. ) : 3 ounces 96.0 cc. 

Powdered Potassium Alum . ; 1 ounce 30.0 grams 


Dissolve in the order given. 
If it is desired to mix a stock hardener solution, use: 


Acid Hardener Stock Solution [ Bor ayara | 


F-la 
Avoirdupois Metric 
Water (about 125° F.) AEN C.) 5S 56 ounces 1700.0 cc. 
Sodium Sulphite (E. K. Co.) . : 16 ouncss 480.0 grams 
*Acetic Acid (28% pure) ‘(E. K. Co. ) ‘ 48 ounces 1500.0 cc. 
Powdered Potassium Alum . : 16 ounces 480.0 grams 
Cold water to make he Bi Be Nae 1 gallon 4.0 liters 


*To make 28% acetic acid from Foor acetic acid, dilute three parts of glacial 
acid with eight parts of water. 


For use, add 1 part of cool stock solution slowly with 
stirring, to 8 parts of a 25% cool hypo solution. 


56 EASTMAN KODAK COMPANY 


To make up the hardener dissolve the chemicals in the | 
order given above. The sodium sulphite should be dissolved 
completely before adding the acetic acid. After the sulphite- 
acid solution has been mixed thoroughly, add the potassium 
alum with constant stirring. If the hypo is not thoroughly 
dissolved before adding the hardener a A precipitate of sulphur 
is likely to form. 


Chrome Alum Fixing Bath for Formula 


Films and Plates | F-16 
Solution A 
Avoirdupois Metric 

Hypo ao 2 pounds 960.0 grams 
Sodium Sulphite (E. K. Co D Ree ae 2. ounces 60.0 grams 
Water tomake .... . . . 96 ounces 3.0 liters 

Solution B 
Water ... ~ . » «+ $2). “ounces 11-0 Titer 
Potassium Chrome Alon hens ae 2 ounces 60.0 grams 
Sulphuric Acid, pure concentrated . 14 ounce 8.0 cc. 


Pour B solution into A solution slowly while stirring A 
rapidly. This formula is especially recommended for use in 
hot weather but it loses its hardening properties in a few days 
either with or without use and therefore should be used as 
soon as possible after mixing. 


3 3 pre Formula 
Motion Picture Fixing Bath [ F-2 
Avoirdupois Metric 
Water A iG re roe eee 1 gallon 4.0 liters 
Hypo . . cod ee 2 pounds 960.0 grams 


When Aor neeee add the following cool hard- 
ener solution slowly with stirring to the cool hypo solution. 


Water .. oh ces 4 ounces 128.0 cc. 
Sodium Sulphite (E. K. Go. so . . 175 grains 12.0 grams 
*Acetic Acid (28% pure) (E. K. Co.) . 2%, ounces 72.0 cc. 
Powdered Potassium Alum .... . 350 grains 24.0 grams 


Dissolve in the order given following the directions for 
mixing the stock hardener (Formula F-1a) given on page 55. 


awe m Formula 
Deep Tank Fixing Bath for Roll Films F-14 
Avoirdupois Metric 
Water oo. ohne ees) eae Lees 1 gallon 4.0 liters 


Hypo . res oe here ok soy 2 pounds 960.0 grams 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY = 57 


When thoroughly dissolved, add the following cool 
hardener slowly with constant stirring to the cool hypo 
solution. 


Avoirdupois Metric 
AWEEY ER <0 ge) Se" Ne 13 ounces 400.0 cc. 
Sodium Sulphite (E.K.Co.) .... 1 ounce 30.0 grams 
*Acetic Acid (28% pure) (E. K. Co.) . 612 ounces 208.0 cc. 
Powdered Potassium Alum... 2 ounces 60.0 grams 


*To make 28% acetic acid from glacial acetic, take 3 parts of glacial acid and 
add 8 parts of water. 


Dissolve in the order given following the directions for 
mixing the stock hardener (Formula F-1a) given on page 55. 


és Formula 
Hypo Test Solution [ HE-1 | 
Avoirdupois Metric 
Potassium Permanganate. .. . 4 grains 0.3 gram 
Sodium Hydroxide (Caustic Soda) . 8 grains 0.6 gram 
Water (distilled) to make... . 8 ounces 250.0 cc. 


Take 8 ounces (250 cc.) of pure water in a clear glass and 
add % dram (1 cc.) of the Hypo Test Solution. A film, or 
two or three prints should then be taken from the wash 
water and allowed to drip into the glass containing the Hypo 
Test Solution. 


If a small percentage of hypo is present the violet color 
will turn green, and with larger concentrations of hypo the 
green color will change to deep yellow. In either case the 
film or prints should be returned to films or the wash water 
and allowed to remain until further tests prove that the 
hypo has been eliminated. This is shown by the violet color 
remaining unchanged when drippings are added to the glass 
containing the Hypo Test Solution. 


REDUCERS 
Persulphate Reducer Formula | 
Stock Solution R-1 
Avoirdupois Metric 
WWHECE tae hile re Oh fe 32 ounces:’-'1.0 liter, 
Ammonium Persulphate . ... . 2 ounces 60.0 grams 
Sulphuric Acid C.P. . ... . 34, dram 3.0 cc. 


For use, take one part of stock solution and two parts of 
water. 


When reduction is complete, immerse in an acid fixing 
bath for a few minutes, then wash. 


58 EASTMAN KODAK COMPANY 


Permanganate Reducer Formula 
Stock Solution A R-2 
. Avoirdupois Metric 
Water 2. oe. 6 OG we ue) SS UN SZ SOU Cesar eens 
Potassium Permanganate. . . . 134 ounces 52.5 grams 


Stock Solution B 
Cold water .- o.« «© » « »« 32 ounces 1.0 liter 
Sulphuric Acid CG. P. . . .. 1 ounce 32.0 cc. 


For use, take stock solution A, 1 part, stock solution B, 2 
parts, water 64 parts. When the negative has been suffi- 
ciently reduced, immerse in a plain hypo solution, or in a 
fresh acid fixing bath for a few minutes, to remove yellow 
stain, after which wash thoroughly. 

The best method of dissolving the permanganate crystals 
in Solution A is to use a small volume of hot water (about 
180° F.) (82° C.) and shake or stir the solution vigorously 
until completely dissolved; then dilute to volume with cold 
water. When preparing Stock Solution B, a/ways add the 
sulphuric acid to the water slowly with stirring and never the 
water to the acid, otherwise the solution may boil and spatter 
the acid on the hands or face causing serious burns. 


Farmer’s Reducer Formula 
Solution A R-4 
Avoirdupois Metric 
a Potassium Ferricyanide . . . . 15 grains 1.0 gram 
Watetfn. 22 2 ee 1 ounce 32.0cc. 


Solution B 
Hypo ho ia ene 1 ounce 30.0 grams 
Water... - .« . « . 32 ounces 1.0 liter 


Add A to B and immediately pour over the negative to 
be reduced. The formula should be prepared immediately 
before using as it decomposes rapidly after mixing together © 
the A and B solutions. When the negative has been reduced 
sufficiently, wash thoroughly before drying. 


Proportional Reducer Formula | _ 
Stock Solution A R-5 
Avoirdupois Metric 

Water 2.02. a ng ee ee 
Potassium Permanganate. . . . 4 grains 0.3 gram 
Sulphuric Acid (10% solution) . . ounce 16.0 cc. 

Stock Solution B 
Water .. . .« . « 96 ounces 3.0 liters 


Ammonium Persulphate . . . . 3 ounces 90.0 grams 


a 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY — s9- 


For use, take one part of A to three parts of B. When 
sufficient reduction is secured the negative should be cleared 
in a 1% solution of sodium bisulphite. Wash the negative 
thoroughly before drying. 


INTENSIFIERS 
Mercury Intensifier [ eal 
(Monckhoven) In-1 


Bleach the negative in the following solution until it is 
white, then wash thoroughly: 


Avoirdupois Metric 
Potassium Bromide Red aes Pre. ou te 34 ounce 22.5 grams 
Mercuric Chloride . . ... . 34 ounce 22.5 grams 
Waterto make .. . . . . 32 ounces _ 1.0 liter 


The negative can be Menon with 10% sulphite solu- 
tion, a developing solution, such as (Formula D-72) diluted 
1 to 2, or 10% ammonia, these giving progressively greater 
density in the order given. To increase contrast greatly, 
treat with the following solution: 


Avoirdupois Metric 
Sodium or Potassium Cyanide ... 1%Z ounce 15.0 grams 
oe) beg: 34 ounce 22.5 grams 
Watertomake .... . . .: 32 ounces _ 1.0 liter 


Dissolve the cyanide and silver nitrate separately, and 
add the latter to the former, until a permanent precipitate is 
just produced; allow the mixture to stand a short time and 
then filter. This is called Monckhoven’s I ntensifier. 


Chromium Intensifier cag 
Stock Solution In-4 
Avoirdupois Metric 
Potassium Bichromate ae d 3 ounces 90.0 grams 
Hydrochloric Acid, me nceitated: : 2 ounces 64.0 cc. 
Watertomake ... . . . . 32 ounces 1.0 liter 


For use, take 1 part of stock solution to Io parts of water. 
Bleach thoroughly, then wash five minutes and re-develop in 
either Nepera Solution 1:4 or in the Elon Hydroquinone 
developer (Formula D-72) diluted 1:2. Then wash thor- 
oughly. Greater intensification can be secured by repeating 
the process. 

Re-Development Intensifier 
Perhaps the simplest method of intensification for nega- 
tives consists of bleaching in the ferricyanide and bromide 
formula used for the sepia toning of prints (Formula T-7a page 
60) and then blackening with sodium sulphide exactly as in 
print toning. 


60 EASTMAN KODAK COMPANY 


TONING FORMULAS | 
Sepia Toning—Hypo-Alum Bath [ee 


-la 
Avoirdupois Metric 
Cold water iyo) alt eee ak le aaa Ae ee 90 ounces 2800.0 cc. 
Hypo 3 sen to ee ee ee eee 16 ounces 480.0 grams 


Dissolve thoroughly, and add the following solution: 


Hot water (about 160° F.) (71° C.) 20 ounces 640.0 cc. 
Powdered Potassium Alum .. . 4ounces 120.0 grams 


Then add the following solution (including precipitate) s/ow/y 
to the hypo-alum solution while stirring the latter rapidly. 


Cold water : Le 2ounces 64.0 cc. 
Silver Nitrate Crystals Mase Ate 60 grains 4.2 grams 
Sodium Chloride (Table Salt). On 60 grains 4.2 grams 


After combining above solutions, 
Add water to make 2 ee ae a eas 4.0 liters 


Note: The silver nitrate should be dissolved completely before adding the 
sodium chloride and immediately afterward, the solution containing the milky 
white precipitate should be added to the hypo-alum solution as directed above. 
The solution should be milky white if correctly mixed. 


For use, pour into a tray standing in a water bath and heat 
to 120° F. (49° C.). Prints will tone in 12 to 15 minutes. If 
boiling water 1s used, for mixing the toning bath, or if the 
order of mixing is changed or if the hypo-alum bath is not 
stirred when adding the white precipitate, the bath will turn 
a dirty gray or black. The bath should never be heated 
higher than 130° F. (54° C.) otherwise blistering, staining and 
non-uniform toning will result. 


Re-Developing Stock Solution | 


No. 1—Stock Bleaching Solution T-7a 
Avoirdupois Metric 
Potassium Ferricyanide . .. . 2142 ounces 75.0 grams 
Potassium Bromide i Tepes rh 2144 ounces 75.0 grams 
Potassium Oxalate . .... . 61, ounces 195.0 grams 
Acetic Acid (28% pure) rem sgamene 3 1144 ounces 40.0 cc. 
Water... . . . 64 ounces 2.0 liters 


No. 2—Stock Re-Developing Solution 
Sodium Sulphide (not Sulphite). . 114 ounces 45.0 grams 
Water... . . . 16 ounces 500.0 cc. 


Prepare Bical: Bath as follows: 
Stock Solution No.1 . . . . . 16 ounces 500.0 cc. 
Water... + +» « » « + +» 416 Ounces -500;0cc: 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 61 


Prepare Re-Developer as follows: 


Avoirdupois Metric 
Stock Solution No.2 ..... . 4 ounces 130.0 cc. 
Water... ‘ pets O20 .0UnCeS 1.0 liter 


Working elon: Teneres print, which should first 
be washed thoroughly, in the Bleaching Bath, allowing it to 
remain until only faint traces of the half-tones are left and 
the black of the shadows has disappeared. This operation 


will take about one minute. 
Note: Particular care should be taken ot to use trays with any iron exposed, 
otherwise blue spots may result. 


Rinse thoroughly in clean cold water as all chemicals must 
be removed. 

Place in Re-Developer Solution until original detail re- 
turns (for about thirty seconds). Immediately after the 
print leaves the Re-Developer, rinse ¢horoughly, then immerse 
it for five minutes in a hardening bath composed of 1 part of 
the hardener recommended for the acid fixing bath (Formula 
F-1a) (page 55) and 16 parts of water. Remove the print 
from this bath and wash for one-half hour in running water. 
The color and gradation of the finished print will not be af- 
fected by the use of this bath. 


Pe eer 
Stain Remover a 


Developer or oxidation stain may be removed bee at 
hardening the film for 2 or 3 minutes in a 5% formalin solu- 
tion, then washing for 5 minutes and bleaching in: 

Stock Solution A 


Avoirdupois Metric 
Potassium Permanganate. . . . 75 grains 5.3 grams 
Waterto make ... . . . . 32 ounces 1.0 liter 
Sak Solution B 
Sodium Chloride (Table Salt) ies 214%, ounces 75.0 grams 
Sulphuric Acid, pure concentrated . %ounce 16.0 cc. 
Water to make . 32 ounces _ 1.0 liter 


Use equal parts of Aand B. The solutions should not be 
mixed until ready for immediate use since they do not keep 
long after mixing. All particles of permanganate should be 
dissolved completely when preparing Solution A (see hint 
on dissolving permanganate, page 58), since undissolved 
particles are likely to produce spots on the negative. Bleach- 
ing should be complete in 3 or 4 minutes. The brown stain 
resulting from manganese dioxide is best removed by im- 
mersing the negative in 1% sodium bisulphite solution. 
Then rinse well and develop in strong light (not sunlight) with 
any ordinary developer such as Formula D-72 diluted 1 part to 
2 parts of water (see page 53). 


62 EASTMAN KODAK COMPANY 


CHAPTER IX. 


Preparing Solutions 


A solution of any kind is obtained by dissolving a solid or 
a liquid in another liquid (or solid). The substance being 
dissolved is called the solute and the liquid in which it is dis- 
solved is called the solvent. The extent to which the solute is 
soluble in the solvent is called its solubility and when the sol- 
vent will hold no more of the solute it is said to be saturated. 


The degree of solubility of any chemical depends on the 
nature of the solvent and on the temperature, which should 
always be stated. 

If a saturated solution 1s cooled to a lower temperature, 
crystals usually form which settle out until the saturation 
point is reached at that particular temperature, though in the 
case of a substance like hypo, if all dust is excluded, crystals 
do not separate out on cooling and.a so-called super-saturated 
solution is obtained. However, if a small crystal of hypo is 
added to the solution, crystals immediately form and con- 
tinue to grow until the saturation point is reached. The best 
method of preparing a saturated solution, therefore, is to 


dissolve the chemical in hot water, cool to room temperature — 


with shaking, allow to stand, and ‘filter. 


For photographic work, saturated solutions are not 
recommended because of their unreliability relative to the 
concentration of the chemicals used. Solutions of definite 
percentage strength are to be preferred and are always 
specified in Eastman formulas. 


When a chemical is dissolved in water the volume of the 


solution is usually greater than that of the water, because the 
particles or molecules of the chemical occupy a certain space 
when in solution. In case two liquids are mixed, the final 
volume of the liquid is not necessarily equal to the sum of 


the volumes of the liquids mixed; it may be greater or it may 


be less. Thus fifty volumes of alcohol when added to fifty 
volumes of water at 70° F., produce ninety-seven volumes 
of the mixture and not one hundred. Moreover, equal 
weights of different chemicals do not occupy the same volume. 


In photography we are concerned only with the weight or 


volume of each chemical in a fixed volume of the solution, so — 


Peeve NI ARY PHOTOGRAPHIC CHEMISTRY — 63 


that when mixing, the chemical should be dissolved in a 
volume of water appreciably less than that called for in the 
formula and then water added up ¢o the volume stated. 


When mixing photographic solutions, the importance of 
following manufacturers’ instructions issued with formulas 
cannot be over estimated. The quantities and the order of 
ingredients have been established by extensive tests and to 
change them is very apt to affect the useful properties of the 
solutions. 


Weights and Measures 
In photographic practice, solids are weighed and liquids 
are measured either by the Avoirdupois or the Metric system. 


The following tables of weights and measures give all the 
equivalent values required for converting photographic 
formulas: 


Weights and Measures—Conversion Tables 
Avoirdupois to Metric Weight 


Pounds Ounces Grains Grams Kilograms 
16 7000 453.59 0.45359 
0.0625 1 437.5 28.35 0.02835 
1 0.0647 es 
0.03527 15.432 1 0.001 
2.2046 35.274 15432 1000 1 


U. S. Liquid to Metric Measure 


Fluid Fluid Cubic 
Gallons Quarts Ounces Drams Centimeters Liters 
1 4 128 1024 3785.3 3.785 
0.25 1 32 256 946.3 0.9463 
1 8 29.6 0.0296 
0.001 0.004 0.125 1 (60 mins.) 3.696 0.0037 
0.0338 0.271 1 0.001 
0.264 . 1.056 33.8 270.52 1000 1 


When a formula is expressed in grains, ounces, and 
pounds, it may be converted into a metric formula by using 
the following conversion values which take into account the 
difference between 32 ounces and one liter. After a con- 
version has been made, the values obtained should be rounded 
off to give convenient working quantities. The error intro- 
duced in rounding off a value should not be greater than 5 
per cent and the ratio between chemicals such as Elon and 
hydroquinone, or carbonate and sulphite should not be 
changed. 


64 EASTMAN KODAK COMPANY 


Solid Conversion Values 
Grains per 32 ozs. multiplied by 0.06834 = grams per liter 
Ounces per 32 ozs. multiplied by 29.924 = grams per liter 
Pounds per 32 ozs. multiplied by 478.8 = grams per liter 


grains per 32 ozs. 
ounces per 32 ozs. 


Grams per liter multiplied by 14.6 
Grams per liter multiplied by 0.03335 


es |e 


Grams per liter multiplied by 0.002085 = pounds per 32 ozs. 
Liquid Conversion Values 
Ounces (liquid) per 32 ozs. multiplied by 31.22 = cubic centimeters per liter 
Quarts per 32 ozs. multiplied by 1000 = cubic centimeters per liter 
Cubic centimeters per liter multiplied by 0.032 = ounces (liquid) per 32 ozs. 
Cubic centimeters per liter multiplied by | 0.001 = ounces (liquid) per 32 ozs. 


Thus a developer formula for a 42 gallon tank would be — 
converted as follows: 


Formula 
Water fe se) Le a, a 
Elon eT ae ON ee Pe 1 ounce 25 grains 
Sodium Sulphite . . . . . . . £=52ounces 
Potassium Metabisulphite . . . . lounce 272 grains 
Hydroquinone’) (223.0 6 34-3 eee 4ounces 86 grains 
Pyro rrr ieee Oe ey ayers Ce 
Sodium Carbonate . . . . . . . 27 0unces 358 grains 
Potassium Bromide. . .... . 260 grains 
Add waterto make. .... . . 42 ¢allons 
t Conversion 

Direct Rounded-off 
Water. ee Se eee 20.0 liters 20 liters 
Elen 5 48 ey ee ee ee 32 grams 
Sodium Sulphite - >. . « 5 1556.1 grams 1560 grams 
Potassium Metabisulphite ... 48.5 grams 49 grams 
Hydroquinone . .. . . . '125.6 grams 126 grams 
Pyro NY PEER a? FE 300 grams 
Sodium Carbonate . . . . . 831.4 grams 835 grams 
Potassium Bromide. . .. . 17.8 grams 18 grams 
Add water to make. . . ... 168.0 liters 168 liters 


To convert a metric formula into an avoirdupois formula, 
the process should be reversed using the values given in the 
second part of the foregoing conversion table. Values in 
grains should be rounded off to the nearest quarter ounce, 
whenever it is possible to do so without introducing an error 
greater than 5 per cent. 

It is often recommended to dissolve, say, 10 parts of a solid 
in 100 parts of water. In the case of liquids, parts should be 
taken as meaning units of volume and in the case of solids as_- 
units of weight. A “part”? may, therefore, mean anything 
from a grain to a ton, or a minim to a gallon so long as the 
other quantities are reckoned in the same units of weight or 
volume. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 65 


Thus: 
For use, take For use, take 
Solution A...... 3 parts\ va {Solution A...... 15 OZs. 
Solution B..,... 1 part { ™ a a TSolution By i355: 5 ozs. 


If the formula contains both solids and liquids, if ounces 
(liquid) and ounces (solid) are substituted for “parts,” the 
error involved falls within permissible limits. 


Example: 


Mix one gallon of solution according to the following 
formula: 


Sodium Sulphite 10 parts 
Pyro I part 
Water to make 100 parts 


One gallon equals 128 ozs. Therefore, dissolve 10K 128 + 
100=12 4/5 ozs. of sulphite in water, add 1% ozs. of Pyro, 
and make up to 1 gallon. 


When quantities of chemicals under Io grains or 0.7 gram 
are included in a formula, they are expressed preferably as a 
Io per cent solution to be added as so many drams or cc. 
If less than a dram is required, an even quarter fraction 
thereof ought to be used. This plan avoids expressing the 
volume in “drops,” which is a very uncertain quantity vary- 
ing as much as 150 per cent depending on the way it is meas- 
ured and the specific gravity of the liquid used. The average 
drop from the usual dropping bottle or burette measures 
about one minim or less than one-tenth of a cc. 


Many photographers are accustomed to making up their 
stock solutions of hypo, carbonate, sulphite, etc., by means of 
the hydrometer. This method has the advantage that in case 
the chemical has become moist and contains an unknown 
quantity of water, a definite reading on the hydrometer will 
give a solution of the same strength as if perfectly dry chem- 
icals had been used. When a stock solution is made from 
moist chemicals by weighing, the error caused by the presence 
of water may be as high as 25% or 50%. 


The hydrometer method has the disadvantage that the 
adjustment of a solution to the required strength takes con- 
siderable time, it does not convey any idea as to the percent- 
age strength of the solution, and varies with the temperature. 
For instance, if a stock solution is made with hot water and 
this registers, say, 45 on the hydrometer, on cooling, the 
liquid may register 48 or 50. It is therefore absolutely 


66 EASTMAN KODAK COMPANY 


necessary either to make all readings when the solutions have 
cooled to room temperature, or to prepare a table giving the 
variation of density of each solution with temperature. 


Mixing stock solutions by hydrometer test is not recom- 
mended because it is much simpler to compound these by 
weighing. A subsequent hydrometer reading, however, is 
sometimes a rough check that the solution has been mixed 
correctly. 4 hydrometer test of a mixed developer or fixing bath 
has no meaning whatever. The only way to test a photo- 
graphic solution is actually to process therein the photo- 
graphic material for which it was intended. 


Stock Solutions 


A stock solution is a concentrated solution to which water 
is added before use. 


The limiting strength of solution which it is possible to 
make in any particular case depends on the solubility of the 
chemical, and as the solubility diminishes with temperature, 
a solution should not be made stronger than a saturated solu- 
tion at 40°F.,(4.4°C) otherwise, in cold weather, the substance 
would crystallize out. 


A stock solution of sodium sulphite should be made as 
strong as possible (20% of the desiccated salt) because at such 
a strength the solution oxidizes very slowly and will therefore 
keep, whereas in weaker solution, it combines with the oxygen 
in the air very readily and is then uSeless as a preservative. 


Percentage Solutions 


The percentage strength of a photographic solution in- 
dicates the quantity of the chemical which is dissolved in 
100 ounces or 100 cc. of the solution. A percentage solution 
is prepared by dissolving the specified quantity of the chem- 
ical in a small volume of water and adding water to make 100 
ounces (or too cc.). In the avoirdupois system a 10% solu- 
tion of a solid is made by taking one ounce and making up to 
ten ounces with water. Converting these figures into grams 
and cc. we have, roughly, 30 grams in 300 cc. or a 10% solu- 
tion. 


A 10% solution of a liquid in water is made by taking 
Io ounces or Io cc. of the liquid and adding water to make 
100 ounces or 100 cc. respectively. 

The great advantage of stating the strength of any solu- 
tion in parts per hundred is that a definite mental picture is 
at once created of its relative strength, while by means of a 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 67 


number of stock solutions it is possible to compound certain 
formulas by simply measuring out a definite volume of each 
solution thus dispensing with a balance. Thus supposing we 
have 10% solutions of potassium ferricyanide and of po- 
tasstum bromide already at hand and it is desired to make up 
the following solution: 


Potassium Ferricyanide 6 grams 
Potassium Bromide 2.3 grams 
Water to 1000 cc. 


then it is only necessary to measure out 60 cc. of the ferri- 
cyanide solution, 23 cc. of the bromide solution, and add 
water to make 1000 cc. 


Suppose a formula calls for 0.1 gram. It is impossible to 
weigh this quantity accurately on the usual photographic 
scale, but by measuring out Io cc. of a 1% solution, and 
adding this to the mixture the problem 1s solved. 


Terminology and Arrangement of Formulas 


In the publication of formulas it is convenient from several 
standpoints to adopt a standard volume of solution for tray 
and tank work. Eastman formulas are published, therefore, 
on a 32 ounce or I liter basis or quarter fraction thereof for 
tray purposes, and a 1 gallon or 4 liters or quarter fraction 
thereof for tank purposes. For special purposes such as 
motion picture or photo finishing work it is necessary to use 
larger volumes for tank formulas. 


As a general rule in published formulas the term ‘Cold 
water to make” is always given at the end of the formula. 
This insures dilution to a definite volume, thus yielding a 
known concentration of chemicals each time the formula is 
‘mixed. With many developers a volume of water at about 
125° F. (52° C.) is given at the beginning of the formula, 
sufficient to dissolve all the chemicals. When finally diluted 
to volume with cold water, the solution will usually be at a 
satisfactory working temperature,65° to 70° F-.(18° to 21°C.). 


The term “Stock Solution” should precede every formula 
which needs to be diluted for use. As mentioned previously, 
the order of chemicals in a formula is established carefully 
and should always be followed when preparing a solution. 

Names of chemicals used in Eastman formulas are those 
accepted by standard text books of chemistry. Vague or 
inaccurate terms such as sulphuret of hydrogen, soda crystals, 
liver of sulphur, etc., are eliminated. 


68 EASTMAN KODAK COMPANY 


Apparatus 


For quantities up to four pounds or 2000 grams a double ~ 
pan balance should be used. For still larger quantities a 
platform scale may be used. For preparing small quantities 
of sample developers a small-chemical balance weighing to 
one-tenth of a grain or one-hundredth gram is necessary. 


For small quantities of solution conical glass flasks are the — 
most suitable, for larger quantities enameled buckets. Earth- 
enware crocks are usually unsatisfactory because when the 
glaze cracks, the solutions penetrate the pores and thus con- 
taminate any other solutions subsequently mixed in them. 


A wooden stick or paddle made of spruce or cypress is the 
best form of stirrer, but a separate one should be used for each 
solution to eliminate the possibility of contamination. 
Paddles should be wax impregnated by first soaking in water 
for several hours to open the pores, and then immersing in 
hot paraffin wax which displaces the water. The wax solution 
should be kept very hot so that only a thin surface layer of 
wax is retained when the paddle is withdrawn. 


The paddle may also be used to measure out a definite 
volume of solution in a tank or crock by marking it to cor- 
respond with definite volumes when held vertically. Such 
markings are only applicable, however, to the particular 
tank or crock for which the paddle was graduated, so that a 
separate paddle should be used for each tank or crock unless 
they are of the same shape and capacity. 


Materials for making containers for photographic solu- 
tions vary according to the nature and volume of the solu- 
tions to be used. Trays and small tanks are usually satis- 
factory if made from any of the following: glass, (such as 
Pyrex) enameled steel, reinforced hard rubber, wax impreg- 
nated wood (cypress or spruce), wood lined with a good grade 


of sheet rubber or rubberized cloth, well glazed porcelain, or 


laminated phenolic condensation products. 


Large tanks, vats, and drums should be made of hard 
glazed stoneware, Alberene stone, wax impregnated spruce 
or cypress, or wood lined with chemical lead and “burned” 
seams. Another simple inexpensive method of tank con- 
struction is to flow hot “Oxygenated”’ asphalt over the surface 
of a cypress wood tank. The asphalt should not contain 
phenolic products or chemical fog will be produced in de- 
veloper solutions. 


Te } 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY — 69 


Tin, copper, and zinc, or alloys of these metals will 
usually produce bad fog and stain with photographic devel- 
opers, and are also unsatisfactory for use in fixing baths. 


Mixing Operations 


Chemicals should be weighed and solutions prepared out- 
side the darkroom. Care must be taken when handling such 
substances as hydroquinone, resublimed pyro, potassium 
ferricyanide, etc., not to shake the finer particles into the air, 
otherwise they will enter the. ventilating system and settle 
on benches, negatives and prints, thus causing no end of 
trouble in the way of spots and stains. 


Weigh chemicals on pieces of paper and after transferring 
to the mixing vessel do not shake the paper but lay it in 
the sink and allow water to flow over it, thus dissolving the 
dust. Larger quantities are most conveniently weighed in 
buckets. e 


For small volumes of solutions a glass graduate marked 
off in ounces should be used; for larger volumes use a bucket 
previously graduated, or mark off the inside of the tank or 
crock used for mixing. When measuring a liquid in a glass 
graduate, place the eye on a level with the graduation mark 
and pour in the liquid until its lower surface coincides with 
this level. Owing to capillary attraction the liquid in contact 
with the walls of the graduate is drawn up the sides, so that 
on viewing sideways it appears as if the liquid has two sur- 
faces. All readings should be made from the lower surface 
and at room temperature because a warm liquid contracts on 
cooling. 


The rapidity with which a substance dissolves in any sol- 
vent depends on its solubility and degree of fineness, the tem- 
perature of the solvent, and the rate of stirring. Since a 
chemical is usually more ‘soluble in hot water than in cold, the 


_ quickest way of mixing a solution is to powder it and dissolve 


in hot water with stirring. In the case of a few substances 
like common salt, which are only slightly more soluble in hot 
than in cold water, the use of hot water is of no advantage. 


Since most solutions are intended for use at ordinary tem- 
peratures (65° to 70° F.) (18° to 21° C.), if hot water is used 
for dissolving, the solution must be cooled again if it is re- 
quired for immediate use, though usually the time taken to 
do this is less than the extra time which would be taken up 
in dissolving the chemicals in cold water. When mixing, 


70 EASTMAN KODAK COMPANY 


therefore, as a general rule, dissolve the chemical in as small 
a volume of hot water as possible, cool and dilute with cold 
water. 

After diluting with water, thoroughly shake the solution 
if in a bottle, or stir if in a tank, otherwise the water added 
will simply float on top of the heavier solution. 


When mixing a solution in a tank, never put the dry chem- 
icals into the tank, but always make sure that they are dis- 
solved by mixing in separate buckets and filtering into the 
tank. 


In the case of anhydrous (dry) salts, such as desiccated 
sodium carbonate, sodium sulphite, a/ways add the chemical 
to the water and not vice versa, otherwise a hard cake will form 
which will dissolve only with difficulty. 


It is necessary to remove from the solution any suspended 
matter such as dirt, caused by the presence of dust in the 
chemicals used, and "also any residife or undissolved particles 
which might settle on the film, plates or paper during de- 
velopment. 


A combination of methods for removing such particles 
is the best and most desirable as follows: 


(a) For volume of solution up to five gallons, filter 
through a fine cloth into a bottle or crock fitted with a side 
tube and pinch cock or screw clamp. In this way the fine 
particles settle but the drainage tube is sufficiently high so as 
not to disturb the sediment. The cloth or muslin should be 
washed thoroughly, otherwise the sizing matter in the fabric 
will be washed into the solution and settle as a sludge. 


(b) For large volumes of solutions such as are used in 
photo finishing work, the best arrangement for mixing is to 
place the chemical room immediately above the developing 
room, and to mix the solutions in large wooden vats, stone- 
ware or enameled tanks connected with the lead piping to 
the developing and fixing tanks in the darkroom below. The 
solutions can then be mixed in advance, allowed to settle, 
and tested, so that only correct solutions pass into the tanks. 


When mixing large volumes of solution in a tank, stretch 
a cloth filter bag over the tank, place the chemicals in the bag 
and allow hot water to flow into it. In this way the chemicals 
are dissolved and the solution filtered at the same time. 
separate bag should be: used for each solution to eliminate 
all risk of contamination. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 71 


Finely divided chemicals should be dissolved separately 
in enameled pails, using a small volume of warm water (about 
wes F.) (52° C.), and then filtered by pouring into the filter 
_ Dag. 

The settling of a semi-colloidal sludge can usually be 
hastened by mixing the solution in warm water, because the 
warmth tends to coagulate the suspension and cause the 
particles to cluster together. Thus if crystals of sodium 
sulphide which are brown, due to the presence of iron, are 
dissolved in warm water the colloidal iron sulphide coagulates 
and settles out rapidly, leaving a perfectly colorless solution. 

When mixing chemicals, if the solution is not filtered or 
if a coarse filter is used, a scum usually rises to the surface 
consisting of fibers, dust, etc., which should be skimmed off 
with a towel. 

When a fixing bath has been used for some time and is 
allowed to stand undisturbed for a few days, any hydrogen 
sulphide gas which may be present in the atmosphere forms 
a metallic looking scum of silver sulphide at the surface of the 
liquid, and on immersing the film this scum attaches itself 
to the gelatin and prevents the action of the fixing bath. 
Any such scum should be carefully removed before use, with 
a sheet of blotting paper or by using a skimmer made of 
several layers of cheese cloth stretched on a frame. 


The Water Supply in Photographic Operations 

Water is the most widely used chemical in photography 
and it is important therefore to know to what extent the 
impurities in it may be harmful to the various photographic 
operations and how these impurities may be removed. 

Impurities in Water. Excluding distilled water, rain 
water, and water from clean melted ice or snow, impurities 
may be present as follows: 

1. Dissolved salts such as bicarbonates, chlorides, and 
sulphates of calcium, magnesium, sodium, and potassium. 

2. Suspended matter which may consist of: 


A. Mineral matter such as mud, iron rust, or free 
sulphur. 


B. Vegetable matter such as decayed vegetation. 


C. Animal matter such as biological growths and 
bacteria. 


72 EASTMAN KODAK COMPANY 


The suspended particles may be of colloidal dimensions 
when they are difficult to remove by filtration. 


3. Dissolved extracts usually colored yellow or brown 
from decayed vegetable matter and the bark of trees. 


4. Dissolved gases such as air, carbon dioxide, and hy- 
drogen sulphide. 

Impurities in the water supply are not responsible, how- 
ever, for as many troubles as is usually supposed. If develop- 
ing solutions are mixed with warm water (about 125°F.) 
(52° C.) and allowed to stand over night, any precipitate or 
suspended matter will settle and the clear supernatant liquid 
may be drawn off for use. The presence of calcium and other 
salts is sometimes beneficial as they tend to retard the swell- 
ing of the gelatin coating of films, plates, and papers during 
washing. This is of particular advantage 1 in hot weather. 


The only impurities liable to cause serious trouble with 


developers are hydrogen sulphide or soluble metallic sul- 


phides. With such water about 25 grains of lead acetate per 
gallon (0.4 gram per liter) of developer should be added be- 
fore mixing. This removes the sulphides, as lead sulphide 
and any excess lead is precipated in the developer and settles 
on standing. 


No trouble may be anticipated with fixing baths prepared 
with average samples of impure water providing the bath is — 
clarified by settling before use. 


When washing photographic materials little trouble may 
be anticipated with uncolored’ water if the following pre- 
cautions are‘taken: (a) remove all suspended matter by filter- 
ing, either by means of commercial filters or by placing two 
or three layers of cloth over the water outlet; (b) remove 
thoroughly all excess moisture from the films, plates or paper 
before drying. 


Water which even after filtering 1s colored brown is very — 
apt to cause staining of the highlights of paper prints. It is 
a difficult matter to remove economically the coloring matter 
from such waters and each case usually requires specific 
treatment. 

Further information on the water supply is contained in 
a pamphlet on that subject obtainable on request from the 
Service Department, Eastman Kodak Co., Rochester, N.Y. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 73 


How to Mix Developing Solutions 
3 developer usually contains four ingredients as follows: 


The developing agent (Elon, hydroquinone, pyro, 
Bee réchenol, Aaa 


2. The alkali (carbonates and hydroxides of sodium, 
potassium, lithium and ammonium). 


The preservative (sulphites, bisulphites, and meta- 
bisulphites of sodium and potassium). 


4. The _ restrainer (bromides and iodides of sodium 
vy potassium). 


If a developing agent like hydroquinone is dissolved in 
water, the solution will either not develop at all or only very 
slowly, and on standing it will gradually turn brown, because 
of what is called oxidation or chemical combination of the 
hydroquinone with the oxygen present in the air in contact 
with the surface of the liquid. This oxidation product is of 
the nature of a dye and will stain fabrics or gelatin just like 
a dye solution. 

On adding a solution of an alkali such as sodium car- 
bonate, the hydroquinone at once becomes a developer, but 
at the same time the rate of oxidation is increased to such 
an extent that the solution very rapidly turns dark brown, 
and if a plate is developed in this solution it becomes stained 
and fogged. 

If we add a little sodium bisulphite to the brown colored 
solution mentioned above, the brown color or stain is bleached 
out and a colorless solution is obtained. Therefore, if the 
preservative is first added to the developer, on adding the 
accelerator the solution remains perfectly clear because the 
sulphite preserves or protects the developing agent from 
oxidation by the air. 

As a general rule, therefore, the preservative should be 
dissolved first. 

An exception to this rule should be observed with con- 
centrated formulas containing the developing agents, Elon, or 
Roylon. These substances are readily soluble in warm 
water (about 125° F.) (52°C.) and do not oxidize rapidly. 
If the sulphite is dissolved before the Elon, as is the case with 
developers such as hydroquinone, a white precipitate often 
appears especially if the sulphite solution is concentrated. 
This precipitate forms because Elon is a combination of an 
insoluble base with an acid which renders it soluble. When 
the acid portion is neutralized by a weak alkali such as so- 


74 EASTMAN KODAK COMPANY 


dium sulphite, the insoluble base is precipitated. This Elon 
precipitate is soluble in an excess of water and also in a 
sodium carbonate solution with which the base forms a 
soluble sodium salt. When once the Elon is dissolved, how- 
ever, it takes a fairly high concentration of sulphite to bring 
it out of solution again, though only a low concentration of 
sulphite is required to prevent the Elon from dissolving. If 
a precipitate forms on dissolving the Elon and sulphite, this 
will usually redissolve on adding the carbonate and no harm 
has been done. 

Some direction sheets recommend that a portion of the 
sulphite should be dissolved in order to prevent the oxida- 
tion of the Elon, then dissolve the Elon, and then the re- 
mainder of the sulphite. Many workers add a little of the 
solid sulphite to the Elon when dissolving the latter. This 
procedure is quite satisfactory, though if the Elon is dissolved 
alone in water at a temperature not above 125° F. (52°C.), 
and the sulphite dissolved immediately afterwards, little or 
no oxidation products will be formed which would otherwise 
produce chemical stain. 

The alkali (usually carbonate) may be added in one of 
three ways: 

(a) Dissolve the carbonate separately and add to the 
cooled Elon-sulphite solution. There is danger, however, of 
the Elon precipitating before the carbonate is added. 

(b) Add the solid carbonate to the Elon-sulphite solu- 
tion, stirring thoroughly until dissolved. 

(c) After dissolving the Elon, dissolve the sulphite and 
carbonate together, cool and add to the Elon-solution. 

Bromides and iodides are added to a developer to com- 
pensate for any chemical fog produced by the developer, or 
inherent in the emulsion. It is immaterial at what stage the 
bromide is added during the mixing. 

When mixing a developer the following rules should, 
therefore, be followed: 

1. Dissolve the chemicals in the order given unless the mixing 
directions specify changes in the order of solution. \f a formula 
contains both sulphite and bisulphite, it is usual to dissolve 
these together, that is the bisulphite is dissolved in the same 
order as the sulphite. 

2. Dissolve each chemical completely before adding the next. 
If the alkali is added before the crystals of the developing 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY = 75 


agent are dissolved, each crystal becomes oxidized at the 
surface and the resulting solution will give fog. 


3. Mix the developer. at the temperature recommended which 
is usually not above 125° FP. (52° C.). 


4. In the case of desiccated chemicals like sodium car- 
bonate and sodium sulphite, add the chemical to the water and 
not vice versa. 


Two practical methods of mixing are possible, as follows: 


(a) Dissolve all the chemicals in one bottle or vessel by 
adding the solid chemicals to the water in the correct order 
(in the formula the ingredients should be named in the order 
in which they are dissolved). For example, to mix the fol- 
lowing formula: 


Avoirdupois Metric 
Elon Be Sila 545) os¥ains 3.1 grams 
Sodium Suiphite (e. K. Ga. ee eee 114 ozs. 45.0 grams 
Hydroquinone . - ot. 4352 drains 9.5 grams 
Sodium Carbonate (E. K Co. Nite ee 21 ozs. 75.0 grams 
Potassium Bromide. . . . . . 15. grains 1.1 grams 
Watertoomake-.- 5... . . 332 ozs. 1.0 liter 


proceed as follows: 

Dissolve the Elon in 16 ozs. (500 cc.) of water (about 
125° F.) (52°C.), then add the sulphite, and when com- 
pletely dissolved, add the hydroquinone. Finally add the 
carbonate and bromide and cold water to make 32 ozs. 
(1 liter). 

For large quantities the filter bag method should be used, 
the chemicals being placed in the bag and dissolved in ‘the 
above order. 


(b) An alternative method is to dissolve the preservative 
and developing agent in one vessel and the carbonate and 
bromide in another, cool and mix. This method is the safest 
and best for quantity production. 

For example, to mix the following motion picture devel- 


oper, 
Avoirdupois Metric 


Sodium Sulphite (E. K. Co.) . ... 4 Ibs. 1800.0 grams 
Hydroquinone . ere Lo OZSs 390.0 grams 
Sodium Carbonate (E. K. Co. ) eee 4 Ibs. 1800.0 grams 
Potassium Bromide. . .. . . 3 ozs. 90.0 grams 
Pearetite Make 4) 2S) oS 10” gals, 40.0 liters 


proceed as follows: 


Dissolve the sulphite in about one gallon (4 liters) of 
water (125° F.) (52° C.), then dissolve the hydroquinone and 


76 EASTMAN KODAK COMPANY 


filter into the tank. Then add one gallon (4 liters) of cold 
water to the tank, dissolve the sodium carbonate and bro- 
mide in one gallon (4 liters) of hot water and filter this into 
the tank, immediately adding cold water up to ten gallons 
(40 liters). The object of adding cold water to the tank before 
adding the carbonate is to cool off the solution before the 
carbonate is added. 


Mixing Concentrated Developers 


The extent to which a developer may be concentrated is 
determined by the solubility of the least soluble constituent, 
because a stock solution should usually withstand cooling to 
40° F. (4.4° C.) without any of the ingredients crystallizing 
out. (See Table of Solubilities, Page 99.) Usually, the hy- 
droquinone and Elon come out of solution on cooling, but 
this may be prevented by adding wood alcohol or methanol 
in a concentration up to 10%. Dénatured alcohol may be 
used if wood alcohol has been added as the denaturant. 
If a precipitate forms, however, on adding the denatured al- 
cohol to the developer, the denatured alcohol is unfit for use. 


The addition of the alcohol does not prevent the other in- 
gredients such as sodium sulphite from crystallizing out, in 
fact, the alcohol diminishes their solubility and therefore in-— 
creases the tendency to come out of solution. 


A para-aminophenol-carbonate developer is difficult to 
prepare in concentrated form, though by adding a little 
caustic soda the solubility of the para-aminophenol is in- | 
creased and a stronger solution can be thus prepared. 


When preparing concentrated developers it is important 
to observe carefully the rules of mixing, taking care to keep 
the temperature of the solution as low as possible if a color- 
less developer is to be obtained. 


The following formula is a typical example of a concen- 
trated developer and is prepared by dissolving the ingredients 
in the order given: 


Avoirdupois Metric 


Water (about 125° F.) (52° C.) <> 16 Sonal 500.0 cc. 

Elon «beh Se Sei ca REG, Ce ee 5.3 grams 
Sodium Sulphite (E. K.Co.). .. . 214 ozs. 75.0 grams 
Hydroquinone ...... . 3/4 OZ. 22.5 grams 
Sodium Carbonate (E. K. Co.) ... 314 ozs. 105.0 grams 
Potassium Bromide a i ale en ok SS aa reer 2.7 grams 
Wood Alcohol ia) mAs, boa Cat ee 44, ozs. 136.0 cc. 


Cold water to make te A ei gis eee 1.0 liter 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 77 


Two-Solution Developers 


A two-solution developer is simply a one-solution de- 
veloper split into two parts, one containing the carbonate and 
bromide, the other containing the developing agent and pre- 
servative so that the developer will oxidize less readily and 
therefore keep well. The reason why it is customary to 
keep a developer like pyro in two solutions, is because pyro 
oxidizes much more readily than Elon or para-aminophenol 
with a given amount of preservative. 


For purposes of mixing only one-solution developers need 
be considered because the same rules regarding mixing apply 
in both cases. 


How to Prepare Fixing Solutions 


Fixing baths may be divided into the following classes: 

1. Plain hypo solutions. 

2. Acid hypo solutions consisting of hypo with the addi- 
tion of sodium bisulphite, potasstum metabisulphite, or 
sodium sulphite with acid. 

3. Acid hardening hypo solutions. 

1. No difficulty is usually experienced when mixing a 
plain hypo solution. When mixing a quantity of solution 
in a tank, the filter bag method (page 70) should be used and 
the hypo dissolved in warm water because the temperature 
drops considerably while the hypo is dissolving. If a scum 
forms on the surface of the solution on standing this should 
be removed by drawing the edge of a towel or blotter across 
the surface. 


If a wooden cover is used for the tank, it should be water- 
proofed to prevent the formation of fungi which produce acid 
substances that turn the fixing bath milky. Waterproof- 
ing may be accomplished by dipping the cover several times 
in a nitrocellulose lacquer solution such as the Eastman 
No. 5119 Lacquer. ‘The cover should be allowed to dry 
between each dipping; and it should be examined subse- 
quently at intervals and redipped when necessary. An alter- 
native method is to wax impregnate the wood by first thor- 
oughly soaking the cover in water for several days to swell 
the pores and then immersing it in a suitable container hold- 
ing hot paraffin wax. The wax should be kept hot during the 
treatment and any excess wiped off with a cloth. Wax im- 
eo covers have the disadvantage that they are quite 

eavy. 


78 EASTMAN KODAK COMPANY 


The slime which occasionally forms on the inner surface 
of wooden tanks can be removed by washing the tank out 
at intervals with sodium hypochlorite solution as described 
in the section on ‘Cleaning Containers for Photographic 
Solutions,” page 97. 


A plain fixing bath, however, is seldom used because it 
gradually becomes alkaline from an accumulation of alkali 
carried over by prints and films from the developer and this 
tends to soften the gelatin, while the image continues to 
develop in the fixing bath, so that if two prints stick together, 
more development takes place at the point of contact, causing 
uneven development. If the bath is acid, the acid kills or 
neutralizes the alkali in the developer carried over, thus pre- 
venting unevenness. 


2. All acid fixing baths contain ae sodium bisulphite, 
potassium metabisulphite, or a mixture of sodium sulphite 


and a weak acid, and the following directions for mixing © 
should be followed: 


a. Do not add the bisulphite or acid sulphite solutions 
to the warm hypo solution or the hypo will turn milky. The 
solutions should be quite cold when mixed. 


b. On keeping, an acid hypo solution gradually becomes 
milky, so that a stock solution of the sodium bisulphite, etc., 
should be kept and added to the plain hypo stock solution as 
required. For general purposes 1% ozs. (45 cc.) of a 50% 
sodium bisulphite solution is added to 32 ozs. (1 liter) of a 
35% hypo solution. If any considerable excess. over this 
amount is added, the hypo rapidly turns milky especially in 
warm weather, owing to the liberation of sulphur. 


A satisfactory bisulphite fixing bath has the following 
composition: : 
Bisulphite Fixing Bath 

Avoirdupois Metric 


Hypo . . eit 8 ounces 250.0 grams 
Sodium Sulphite (E. K. Ga. ye . . 150 grains 10.5 grams 
Sodium Bisulphite (E. K.Co.) . . 75 grains 5.3 grams 
Water to make .. Pg ea 32 ounces 1.0 liter 


3. Acid hardening Bet baths are prepared by adding to : 
hypo an acid hardening solution which contains the following 
ingredients: 


a. An acid such as acetic, citric, tartaric, lactic, malic, 
maleic, sulphuric, etc., which stops development. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY = 79 


b. A hardening agent such as potassium alum, potassium 
chrome alum or formaldehyde, 40%. 


c. A preservative such as sodium sulphite or sodium 
bisulphite. The latter acts as a preservative in two ways: 
It prevents the formation of sulphur by the action of the acid 
on the hypo, while it also prevents the developer carried over 
into the fixing bath from oxidizing and turning brown. 


Prepare the acid hardening solution as a separate stock 
solution and add this to the hypo solution as required. 


The order of mixing is important, as follows: 


When mixing in one vessel, first dissolve the sulphite 
in warm water (about 125° F.) (52° C.), then add the acid 
and then the potassium alum. It is sometimes recommended 
to reverse the process, namely, dissolve the alum first, add 
the acid, and then the sulphite, but the alum dissolves more 
readily in the acid-sulphite solution. 


Another method 1s to dissolve the alum and sulphite in 
separate solutions, cool, add the acid to the sulphite solution 
and then add the alum solution. 


The hypo should be cool and dissolved completely before 
adding the cool hardener; otherwise sulphur is likely to be 
precipitated. 


If the order of mixing is reversed and the alum added first 
to the sulphite a white sludge of aluminum sulphite is formed 
which dissolves with difficulty when the acid is added. There- 
fore, if after mixing, the hardener is milky and a sludge settles 
out it is due to a relative insufficiency of acid, that is, the 
acid used was either not up to strength or too much alum or 


sulphite was added. 


With other hardening baths the order of mixing 1s usually 
the same. 


Fixing baths containing chrome alum as the hardening 
agent usually have sulphuric acid as the active acid. (See 
Formula F-16, p. 56.) No difficulty should be found in mix- 
ing the bath providing care is taken in adding the sulphuric 
acid to the chrome alum solution. The acid should be poured 
slowly down the side of the mixing vessel while stirring the 
chrome alum solution to insure thorough mixing, since this 
acid is very heavy and will sink to the bottom if not mixed 
well. Water should never be added to the sulphuric acid or 
the solution may boil and spatter some of the actd on the hands 
or face causing serious burns. 


80 EASTMAN KODAK COMPANY 


Storage of Chemicals 


Chemicals should be stored in well corked or well stop- 
pered jars in a cool, dry place because most chemicals are 
affected by air, which contains oxygen, carbon dioxide gas and 
moisture. 


(a) Oxygen readily attacks such substances as sodium. 


sulphite, especially in the presence of moisture, converting it 
into sodium sulphate, which is useless as a preservative. 
With crystalized sodium sulphite the sodium sulphate forms 
on the outside of the crystals as a powder, which may be 
washed off and the crystals dried. It is not easy to detect 


sodium sulphate in desiccated sulphite except by chemical 


tests. 


Other substances which combine with oxygen and are, 
therefore, said to be oxidized, are sodium bisulphite and 
potassium metabisulphite and all developing agents such as 
pyro, hydroquinone, etc., which turn more or less brown, the 
extent of the color roughly indicating the degree of oxidation. 


(b) Carbon dioxide gas combines with substances like 
caustic soda and caustic potash, converting them into the 
corresponding carbonated alkalis which are less reactive. If 


sodium hydroxide is kept in a stoppered bottle the stopper — 


usually becomes cemented fast by the sodium carbonate 
formed, so that it should be kept in a waxed corked bottle. 
Owing to the solvent action of the caustic alkalis on glass the 
inside of the glass bottle containing caustic or strongly car- 
bonated solutions becomes frosted, though the quantity of 
glass thus dissolved away will usually do no harm. 


(c) Certain chemicals have a strong attraction or affinity 
for the moisture in the atmosphere and gradually dissolve 
in the water thus absorbed, forming a solution. This phe- 


nomenon is termed “‘deliquescense’” and the chemicals are 


said to be “‘deliquescent.” Familiar examples are ammonium 


thiocyanate, potassium carbonate, sodium sulphide, uranyl — 


nitrate, sodium bichromate, etc., which should be stored 
in corked bottles and the neck dipped in melted paraffin 
wax. 


a 


As mentioned earlier, it is dificult to prepare a solution of 


definite percentage strength from a chemical which has 
deliquesced, though it is usually sufficient to drain off the 
crystals, or to use a hydrometer, referring to a table giving the 
hydrometer readings in terms of percentage strength. 


-_ 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 81 


(d) While some chemicals absorb moisture as above, 
others give up their water of crystallization to the atmos- 
phere, and therefore lose their crystalline shape and fall to a 
powder and are then said to “efHoresce,” the phenomenon 


‘being termed “‘efflorescence.” Some crystals do not contain 


any water and therefore cannot efHoresce. 


A very dry atmosphere is suitable, therefore, for storing 
deliquescent salts but not for efHorescent salts. The only 
way to store chemicals is to isolate them from the air by suit- 
ably sealing. 


(e) Some chemicals are decomposed by long exposure 
to light, especially sunlight. Such chemicals usually change 
more rapidly when made up as a solution than when stored 
in the solid form. Silver nitrate is probably the outstanding 
example. Crystals of this salt darken in light and a solution 
will darken quite rapidly. The remedy is to store both the 
solid and liquid in dark brown bottles. Potassium iodide 
solutions often turn a deep yellow color because of the libera- 
tion of free 1odine. Nitric acid sometimes turns yellowish 
brown on standing in white bottles for long periods of time. 
Potassium ferricyanide solution turns blue on prolonged 
exposure to light owing to the formation of Prussian blue. 


Stock solutions and developers should be stored in either 
hard glazed earthenware crocks, large glass bottles, wooden 
vats, or tanks of resistant material, and so arranged that the 
liquid may be drawn off at the side and near the bottom. 
(See section on “Apparatus,” page 68.) 


In case a solution such as Pyro has to be stored for a long 
time and withdrawn at intervals, an absorption bottle con- 
taining alkaline pyro may be fitted at the intake which ab- 
sorbs oxygen from the air as it enters the bottle on withdraw- 
ing part of the solution. 


Hard glazed earthenware crocks are most satisfactory for 


storage of stock solutions of developers and fixing baths for 


volumes of 5 gallons (20 liters) or more. The crocks should 
be fitted with waxed wooden covers. The method of attach- 
ing the outlet isimportant. If not supplied with a hole slight- 
ly above the base, the crock should be drilled. A right- 
angled lead tube should then be inserted through a rubber 
stopper and the tube and stopper fastened securely to the 
jar by passing a brass or monel metal band around it. A 
short length of pure gum rubber tubing may be fastened over 
the end of the lead tube and closed by means of a screw 


iD) 


clamp. Large ee de Ree ‘mot 
breakage, may also be used as containers, and tl 
‘fitted in a similar way with either glass or lead 


A battery of stock solution bottles or crocks 
ranged on lead covered shelves under which a larg 
placed, or, the floor may be so arranged as to form 
that in case of accidental breakage no serious damag 
This precaution is of special importance in the cas 
solutions, which might percolate into various root 
studio or laboratory and inoculate them with | 3 
causing an epidemic of spots. - 


Wooden storage tanks may be reinforced 8 
by coating their inner surfaces with ° ‘Oxygenated 
as described under the section on “Apparatasy ag 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY — 83 


CHAPTER xX 


Using Solutions 


Photographic solutions, especially developers, vary con- 
siderably in their period of usefulness or time during which 
they may be utilized effectively to process exposed films and 
papers. This “useful life’ as it is sometimes called, is, there- 
fore, an important property of a solution and should be 
studied by everyone handling photographic materials. There 
are a good many factors which influence the useful life of a 
solution, such as whether it receives intermittent or continu- 
ous use, the extent of the surface exposed to the air, the tem- 
perature, the nature and reactions of the chemical constitu- 
ents, and the manipulative procedure used in handling films 
or prints in the bath. 


The average photographic solution is usually discarded 
as soon as its working rate is reduced to an impractical time 
period. Methods of reviving developers are somewhat vague 
and much work remains to de done on this subject. In the 
case of certain tank developers it is customary to adda 
replenisher solution several times before the developer 1s 
discarded. More is known about the revival of fixing baths 
however, but with these solutions it is usually safer and more 
economical except in specific cases to discard them after a 
certain period of usefulness than to bother with revival. 


Conversely, some workers go to the other extreme and use 
solutions for several years merely by withdrawing a part of 
the used bath and adding fresh solution at regular intervals. 
Though this practice has some merit it is generally to be 
condemned since most photographic solutions accumulate cer- 
tain reaction products that greatly reduce their efficiency 
and in addition may have harmful effects. 


This chapter contains a summary of the characteristics of 
developers and fixing baths both with and without use, 
a discussion of troubles, the effect of temperature on solu- 
tions, and methods of cleaning containers for photographic 
solutions, 


The Useful Life of Developers 


Without Use. If a freshly mixed developer (prepared 
with water boiled to free it from dissolved air) 1s stored 1n a 


84 EASTMAN KODAK COMPANY 


completely filled and stoppered or wax-corked bottle, it will 
keep almost indefinitely even in the light. Under ordinary 
conditions of storage, the bottle or vessel contains more or 
less air. Also, when an ordinary cork or a non-airtight cover _ 
is used, the surface of the developer is continually in contact 
with air, the oxygen constituent of which oxidizes the de- 
veloping agent and sodium sulphite present. This results 
in a lowering of the developing power in direct proportion 
to the amount of oxidation of the developing agent, which is 
accelerated as the preservative or sodium sulphite becomes 
oxidized also. 


The oxidation products of developing agents are usually 
colored so that the developer on keeping frequently turns - 
brown. In the presence of sodium sulphite, however, the — 
oxidation products of hydroquinone consist of hydroquinone 
mono- and disodium sulphonates which are colorless. The 
fact that an old Elon-hydroquinone developer is colorless i 1S, 
therefore, no indication that the original developing power is 
unimpaired. An oxidized Elon or Elon-hydroquinone de- 
veloper also frequently fluoresces strongly. 


In some cases when an Elon-hydroquinone developer 
gives slight developer fog when freshly mixed, the fogging © 
tendency disappears on standing. This may be due to the 
anti-aerial fogging action of the developer oxidation products — 
which are produced on keeping. (See paragraph on fog under 
“Developer Troubles,” page 85.) 


A solution of a developing agent, such as pyro, to be 
stored for a considerable time, will keep best in the presence 
of an acid sulphite such as sodium bisulphite rather than 
sodium sulphite which is slightly alkaline. It is always 
preferable therefore to prepare such a developer as two solu-— 
tions: one containing the developing agent and sodium bi- 
sulphite, and the other the carbonate and bromide, and to 
mix these solutions as required for use. A plain solution of 
sodium sulphite oxidizes readily in contact withair at a con- — 
centration below 10%, but above this concentration it 
oxidizes very slowly. Stock solutions containing sodium 
sulphite alone or in combination with a developing agent 
should be prepared, so that the concentration of the sulphite 
is around 10% for maximum keeping properties. Owing to- 
the relative insolubility of Elon in a sodium sulphite solu- 
tion, it is not possible to prepare such stock solutions with 
Elon. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY — 85 


Sodium bisulphite keeps satisfactorily in more dilute 
solutions and is a better preservative than sulphite in the 
absence of carbonate. It is usual therefore to keep readily 
oxidizable developing agents such as pyro, amidol, etc., by 
mixing with sodium bisulphite. On adding sodium carbonate 
to sodium bisulphite, sodium sulphite and sodium bicarbon- 
ate are formed, so that in compounding a two-solution for- 
mula from a one-solution formula it is necessary to take care 
of this neutralization of the carbonate by using an extra 
quantity. 

Single solution developers containing sodium hydroxide 
or potassium hydroxide do not keep unless well stoppered; 
pure gum rubber stoppers being most suitable. 


With Use. During development, several reactions are 
taking place: (1) The developing agent and sulphite are being 
oxidized by the air; (2) the developing agent is being de- 
stroyed as a result of performing useful work in reducing the 
exposed silver halide emulsion to metallic silver; and (3) oxi- 
dation products of the developer and the by-products, sodium 
bromide and sodium iodide, are accumulating. The bromide 
and iodide and developer oxidation products restrain develop- 
ment while the oxidation products prevent aerial fog. The 
restraining action of the bromide and iodide is analogous to 
cutting down the exposure, so that with an old developer it 
is not possible even on prolonged development to get the 
ultimate result out of an under-exposure. 


The time required to produce a definite contrast increases 
as a developer is used, and the solution ceases to be useful 
when the time required for this exceeds the maximum time 
which can be allotted for the developing operation. A deep 
tank developer, for example, is therefore discarded for one or 
more of the following reasons: (1) The time for complete 
development is excessively long; (2) The solution stains or 


fogs emulsions badly; and (3) The accumulation of by-prod- 


ucts is so great that shadow detail is lost even with full de- 
velopment. 


Developer Troubles 


The Developer Gives Fog. Fog is the chief trouble caused 
by faulty mixing. It may bea result of violation of the rules 
of mixing such as dissolving the carbonate before the sul- 
phite, mixing the solution too hot, omitting the bromide, 
adding too much carbonate or too little sulphite, the use of 
impure chemicals, etc. 


86 EASTMAN KODAK COMPANY 


With certain developers, notably those containing Elon 
and hydroquinone, a form of fog, known as aerial fog, is 
produced when film wet with developing solution is exposed 
to the air. Motion picture positive film developed on a reel 
is especially sensitive to aerial fog. It may be prevented by 
adding about 5% of old developer to the freshly mixed de- 
veloper. This is more effective than increasing the concen- 
tration of bromide above the normal quantity added. The 
oxidized developer probably acts as an anti-fogging agent 
thus reducing the tendency for fog formation. 


Negatives developed in a developer containing an excess 
of sulphite or one containing hypo or ammonia may show 
dichroic or green fog. his appears yellowish-green by re- 
flected light, and a pink color by transmitted light. It is 
usually caused when the dissolved silver salts, under certain 
conditions, are reduced to metallic silver in a very fine state 
of subdivision, particularly in the shadow portions of the 
negative where no bromide is liberated during development. 
Fine grained emulsions are most susceptible to this form of 
fog. Information on prevention and removal of dichroic 
fog is given in an article “Stains on Negatives and Prints,” 
obtainable from the Service Dept., Eastman Kodak Ca 
Rochester, N.Y. 


Some deep tank developers may begin to fog a short time 
after they have been put into use. When this occurs it 
usually can be traced to the presence of sulphide in the solu- 
tion caused by the action of bacteria which reduced the sul- 
phite in the developer to sulphide. The fog may be cleared 
up by putting some waste film or plates through the solution 
or by adding a small quantity of lead acetate to the developer. 
The bacteria or fungi usually grow in the slimy deposit which 
accumulates on the inner walls of the tank. This deposit may 
be removed by sterilizing the container occasionally with 
bleaching powder. (See section on Cleaning Containers for 
Photographic Solutions, page 97). 


The Solution is Colored. As a general rule, the developer 
when mixed should be colorless and if colored it should be 
suspected as being likely to give fog. In the case of a pyro 
developer mixed with bisulphite, which contains iron, an 
inky substance is formed as a combination product of the 
iron and the pyro, and this imparts a dirty bluish-red color 
to the solution although photographically it is harmless. 
If a two-solution pyro developer is mixed in dirty vessels 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 87 


the B solution (which contains the carbonate and bromide) 
may be colored brown by the presence of a little pyro. 


_ The Solution Does Not Develop. Omission of the develop- 
ing agent or the carbonate may usually be suspected if a 
developer does not develop. 


Precipitation of a White Sludge. If a white precipitate 
settles on standing, this is probably Elon. The precipitate 
may often be redissolved by adding 5% of wood alcohol or 
methanol, but if this is not successful, then the formula 
contains either too much Elon or sulphite or not enough 
carbonate. If it is known that the formula gives a clear 
solution when mixed correctly and should the Elon precipitate 
out during mixing when the sulphite is added, the precipitate 
will usually redissolve on adding the carbonate. If the final 
solution is not colored, no harm will have been done. 


Scum. Scum may be picked up on films or plates from 
the surface of the developer especially if the solution has been 
allowed to stand unused for several days. The scum may 
consist of grease, solid matter, or developer oxidation prod- 
ucts, especially if the developer contains pyro. The scum 
should be removed by passing the edge of a sheet of blotting 
paper along the surface of the solution or by using a skimmer, 
consisting of several layers of cheese cloth stretched over a 
square wire frame. 


Miscellaneous Troubles. Various types of developer stains 
may be produced on films, plates, and papers. These are 
discussed in detail in the article referred to under “Fog,” 
page 85. When a developer solution is not agitated suffi- 
ciently during the progress of development, characteristic 
markings are produced. This is occasionally observed with 
film developed on hangers or racks. These markings are 
usually the result of retardation of development along the 
sides of a hanger or rack caused by the accumulation of 
oxidized developer products and sodium bromide. They may 
be prevented by thorough agitation of the holder or rack 
during development. 


The Importance of Rinsing . 


It is important to rinse films, plates and papers after de- 
velopment and before fixation. When a film or print is 
transferred from the developer directly to the fixing bath the 
alkali in the developer retained by the film or print neutralizes 
some of the acid of the fixing bath. The addition of developer 


88 EASTMAN KODAK COMPANY 


also gradually destroys the hardening properties of the fixing 
bath. Therefore, by removing as much developer as possible 
from the film or print by thoroughly rinsing in water or an 
acid rinse bath for Io or 20 seconds, the life of the fixing 
bath is very much prolonged, while the tendency for stains 
and blisters to form is very much reduced. 


Rinsing and Hardening Films or Plates. In warm weather 
it is only possible to rinse films or plates for one or two 
seconds; otherwise the gelatin will soften. If the chrome 
alum hardening bath (Formula SB-3, page 55) is used, rinsing 
in water may be omitted although a previous rinse for a few 
seconds in water will prolong the life of this bath also. Films 
or plates should be agitated for several seconds after putting in 
the hardening bath; otherwise a chromium scum, which 1s diffi- 
cult to remove 1s apt to form on the film. This scum is composed 
of chromium hydroxide and is produced by the reaction be- 
tween the chrome alum and the alkaline developer carried 
over on the film, but it does not form with a fresh bath if the 
film is well agitated on immersion. When the bath becomes 
old, a scum will tend to form even if the films are agitated; 
the bath should then be discarded. Films should always be 
wiped with wetted cotton after washing to remove any possible 
traces of scum, because once the film is dry it is impossible 
to remove it. 

The hardening bath is a blue color by artificial light when 
freshly mixed, but it ultimately turns yellowish-green with 
use. Jt then ceases to harden and should be replaced by a fresh 
bath. 

Rinsing Prints. Thorough rinsing will largely prevent 
staining troubles with prints and will allow a larger number to 
be fixed before the bath sludges. An acid rinse or “short 
stop” bath is strongly recommended instead of a water rinse 
because it arrests development immediately, whereas, when 
rinsing in water, development of the print continues if the 
rinsing is unduly prolonged. 


When handling only a few prints, a rinse of 5 to Io 
seconds is sufficient, but if large batches of prints are being 
processed, the rinsing time should be from 1 to 2 minutes. 
It is important to move the prints and see that they are sep- 
arated while in the acetic acid rinse bath (Formula SB-1 
page 55), and in the fixing bath to insure that the solutions 
have thorough access to all parts of every print. If prints” 
are not rinsed, developer is carried over to the fixing bath and 
the alkali in the developer rapidly neutralizes the acid in the 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY = 89 


fixing bath. When a certain quantity of developer has been 
carried over, a white sludge forms and the bath becomes 
alkaline. Prints fixed in an alkaline bath are likely to be- 
come stained brown. When an acid rinse bath is used, no 
harm is done if the prints are left in the rinse bath Io or 
20 minutes. 

When an acid rinse bath is used, sludging of the fixing 
bath will never occur if the rinse bath is always kept acid. 
A simple method of testing whether the bath 1s alkaline or 
acid is to dip a strip of blue litmus paper in the bath. If 
the paper turns red, the bath is acid, but if it remains blue, 
the bath is alkaline. 


The life of the acid rinse bath (Formula SB-1, page 55) 
used for papers is determined by the quantity of alkali car- 
ried over from the developer, which depends on the quantity 
of carbonate in the developer, the quantity of developer re- 
tained by the print, and the time of draining. With a 1- to 
2-seconds drain and a typical Elon-hydroquinone developer, 
the equivalent of approximately seventy-five 8xIo prints 
per gallon (forty 3% x 5% prints per pint) may be processed 
safely 1 in the acid rinse bath before the bath becomes alkaline. 


The Properties of Fixing Baths 


A plain solution of hypo is seldom used as a fixing bath 
but it is usually used in conjunction with a weakly acid salt 
such as sodium bisulphite, or more commonly with an acid 
hardening solution. The standard hardener contains a pre- 
servative, sodium sulphite, which prevents decomposition of 
the hypo; an acid, usually acetic acid, to neutralize any alkali 
carried over in the film from the developer and thereby 
arrest development; and a hardening agent, either potassium 
alum or chromium alum. 

A satisfactory acid hardening fixing bath should have cer- 
tain properties, namely, a fairly rapid rate of fixation, good 
hardening, a long sludging life, and a long “useful” life. The 
time for fixation is usually taken as twice the time for the 
milkiness or opalescence of the unreduced silver salts to 
disappear. This depends on the strength of the hypo (30% 
to 40% fixes most rapidly), the photographic material tested 
(portrait films fix in about 3 to 5 minutes whereas lantern 
slides clear in 30 seconds to I minute), the temperature of the 
solution (65° F. or 18° C. is recommended), and the degree 
of exhaustion of the solution. 


go EASTMAN KODAK COMPANY 


The hardening properties are influenced by a large number 
of factors. A certain minimum quantity of alum is required 
to give the necessary hardening, while an excess of alum may 
produce too much hardening and induce brittleness. Normal 
fixing baths such as Formula F-1, page 55, are compounded 
carefully to give a hardening of 130° to 170° F. (54° to 77° C.). 
This is determined by immersing a strip of the fixed and 
washed films in water and heating the water slowly until the 
gelatin flows away from the support. For maximum hardening 


using a $-seconds rinse in water between development and 


fixation, and washing one hour in running water after fixation, 
films should be fixed 1 5 to 20 minutes in either fresh or partially 
exhausted fixing baths. 


A fresh chrome alum fixing bath loses its hardening 
properties rapidly whether it is in use or not. For this 
reason a potash alum fixing bath is usually to be preferred 
for long periods of usage. 


A good fixing bath should not sludge | during its useful 
life when used at 65° to 70° F. (18° to 21° C.). Changes in 
temperature of the fixing bath affect the rate of fixation and 
the useful life of the solution. For example, if a film requires 
95 seconds to clear at 65° F. (18° C.), it would take about 60 
seconds to clear at 85° F. (29° C.), but it is dangerous practice 
to allow the temperature of the bath to rise above 70° F. 
(21° C.) as the solution is apt to precipitate sulphur. 


Under tropical conditions where high temperatures pre- 
vail, it is obviously often impossible to keep the temperature 
within this limit, and the fixing bath must usually be re- 
placed oftener. A different technic must therefore, be used 
for tropical processing, where the secret lies in preventing 
abnormal swelling of the gelatin, for once it is swollen it is 
almost impossible to reduce it and to handle the film. For 
more complete information on this subject, the booklet 

‘Tropical Development’”’ should be consulted. This is ob- 
saitable on request from the Service Dept., Eastman Kodak 
Company, Rochester, N. Y. 


The Useful Life of Fixing Baths 


A fixing bath in use becomes exhausted as a result of per- 
forming useful work in fixing out the emulsion. The acidity 
of the bath is being reduced by the developer carried in, 
although at first this tends to favor a longer * ‘sulphurization 
life” or period of time before the bath precipitates sulphur. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY _ g1 


With use, however, the solution finally reaches a point where 
a sludge of aluminum sulphite is precipitated, rendering the 
bath useless. During the first stages of use, the hardening 
properties increase slightly, after which they fall off rapidly. 
a fixing bath is usually exhausted if it froths at the surface, 
or if it becomes milky or sludges throughout the solution. 
The bath may also fix so slowly that there is danger of re- 
moving the films or prints before they are completely fixed. 
When the time of clearing for a slow fixing film exceeds 12 to 
15 minutes, the bath should be discarded. 

The F-1 and the F-16 fixing baths on pages 55 and 56 will 
fixcompletely the equivalent of seventy-five 8 x 1o films or 
plates, and the F-1 formula, one hundred 8 x Io prints per 
gallon, provided a thorough water rinse precedes fixation. 
If a 2- or 3-minute immersion in a suitable hardening bath 
(Formula SB-3 page 55) is given between development and 
fixation, the equivalent of one hundred 8 x to films may be 
fixed per gallon. 

When making prints, if the acid rinse bath (Formula 
SB-1, page 55) is used between development and fixation, the 
fixing bath will not sludge so rapidly and the equivalent of 
one hundred and twenty-five 8 x Io prints may be fixed 
safely per gallon of Formula F-1. 

These figures have been established by careful tests and 
it is recommended that the fixing bath be discarded and re- 
placed by a fresh bath when approximately this number of 
films or prints have been fixed. 


Recovery of Silver from Exhausted Fixing Baths 


An exhausted fixing bath contains dissolved silver salts 
and various methods may be employed to recover the silver 
profitably, providing at least 5 gallons of well exhausted hypo 
are discarded each week. For large volumes of exhausted 
baths (about 100 gallons or more per week) precipitation 
with sodium sulphide is the most economical and rapid 
method. Precipitation with zinc dust is efficient when 
smaller volumes of bath are to be treated, and has the ad- 
vantage that no objectionable fumes of hydrogen sulphide 
are evolved, as in the sulphide process. 

Recovery by means of commercial electrolytic units also 
represents a simple and economical procedure for volumes of 
exhausted baths of less than 100 gallons per week. Electro- 
lytic units give best results when used in a discarded fixing 
bath, rather than in a working bath. 


92 EASTMAN KODAK COMPANY 


Although it is possible for a capable chemist, to so restore 
a fixing bath by desilvering, subsequently clarifying, and 
modifying its composition, that its useful life is prolonged, 
it is just as economical and preferable to prepare a fresh bath. 

Working details of the various methods of silver recovery 
and a discussion of the economics of the processes are given 
in a paper on silver recovery obtainable on request from the 
Service Dept., Eastman Kodak Company, Rochester, N. Y. 


Fixing Bath Troubles 


A. Sludging of the Fixing Bath: A fixing bath occasionally 
turns milky soon after the hardener is added, and some- 
times after being in use for a short time. The milkiness may 
be of two kinds: . 

1. If the precipitate is pale yellow and settles very slowly 
on standing, it consists of sulphur and may be caused by (a) 
too much acid in the hardener; (b) too little sulphite or the 
use of impure sulphite, in which case there is not sufficient 
present to protect the hypo from the acid; (c) high tempera- 
ture. The hardener should only be added to the hypo solu- 
tion when at room temperature. If the temperature of the 
acid fixing bath is over 85° F. (29° C.), it will not remain clear 
longer than a few days even when mixed correctly. Theonly | 
remedy is to throw the bath away and mix fresh solution as 
required. 3 

If a sulphurized bath is used, the sulphur is apt to pene- 
trate the gelatin, and later may cause fading of the image. 


2. If the precipitate is white, and a gelatinous sludge of 
aluminum sulphite settles on standing, it may be caused by 
(a) too little acid in the hardener; for example, supposing 
a formula calls for pure glacial acetic acid and 28% acid is 
used by mistake, then less than one-third the required con- 
centration of acid is present; (b) too little hardener in the — 
fixing bath. When fixing prints, a relatively large proportion 
of the developer is carried over to the fixing bath (unless a 
water or acid rinse bath has been used) which soon neutralizes 
the acid, and therefore increases the tendency for precipita- 
tion of aluminum sulphite. In the same way a fixing bath 
with the correct proportion of hardener, when exhausted, 
still contains alum and sulphite but no acid, and these com- 
bine to form a sludge of aluminum sulphite. 


It is extremely important, to use only the acid specified 
and to know its strength, because trouble is caused if more or 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 93 


less acid is used than 1s called for in the formula. It has been 
found that the hardening properties of an alum-acid fixing 
bath bear a relation to the tendency of the bath to precipitate 
aluminum sulphite. In other words, a bath containing an 
excess of acid (and which therefore may be used for a rela- 
tively long time before fhe aluminum sulphite precipitates), 
does not harden as well as a bath which precipitates when a 
much smaller volume of developer is added. With such a 
bath containing a minimum of acid it is advisable to add a 
further quantity of acid as soon as a slight precipitate ap- 
pears; a satisfactory quantity being about one-half that 
originally present in the bath. 


B. The bath does not harden satisfactorily. Insufficient 
hardening may be a result of (1) the use of inferior alum 
which does not contain the correct proportion of aluminum 
sulphate; (2) the presence of too much acid or sulphite; or 
(3) an insufficient quantity of alum. On varying the propor- 
tions of acid, alum and sulphite in a fixing bath, it has been 
found that the hardening increases as the quantity of alum. 
increases. With increasing quantities of acetic acid, with a 
given quantity of alum, the hardening increases to a maxi- 
mum, beyond which it decreases until the solution does not 
harden at all. A certain minimum quantity of acetic acid, 
however, is necessary to give the fixing bath a fairly long, 
useful life, before aluminum sulphite precipitates, but this 
quantity is usually greater than the quantity which produces 
maximum hardening. With use, therefore, the hardening 
ability of most fixing baths at first increases with the addition 
of developer to a maximum, beyond which the hardening falls 
off rapidly. 

C. Blisters. When the sodium carbonate of the developer 
is neutralized by the acid in the fixing bath, carbon dioxide 
gas is evolved which produces blisters if the gelatin is too soft 
to withstand the disruptive action of the gas. If the fixing 
bath contains an excess of acid and the films are not rinsed 
sufficiently, or if a strongly acid rinse bath is used, blisters 
are apt to be formed. On dry film, blisters appear as tiny 
crater-like depressions when examined by reflected light. 
This trouble is more liable to occur in hot weather, and 
especially when the bath is not hardening sufficiently. 


D. Dichroic Fog. If the fixing bath does not contain 
acid or if it is old and exhausted and contains an excess of 
dissolved silver salts, a stain called dichroic fog is sometimes 
produced on the film. In reflected light, film stained in this 


94 EASTMAN KODAK COMPANY 


way appears yellowish-green and by transmitted light it 
looks reddish-pink. Dichroic fog never occurs in a fresh 
acid fixing bath, or if the film is rinsed before fixing and the 
temperature of the bath is kept at 65° to 70° F. (18° to 21° C.). 
Methods of removal of dichroic fog are discussed in the book- 
let, “Stains on Negatives and Prints” obtainable from the — 
Service Dept., Eastman Kodak Company, Rochester, N. Y. 

E. Scum on Fixing Baths. When a partially exhausted © 
fixing bath is allowed to stand several days without use, the 
hydrogen sulphide gas usually present in the air reacts with 
the silver thiosulphate in the bath and forms a metallic 
appearing scum on the surface of the solution. This scum 
consists of silver sulphide and should be removed by drawing 
the edge of a sheet of blotting paper across the surface of the 
bath, or by using a skimmer made of several strips of cheese-_ 
cloth stretched over a wire frame. 

A white scum consisting of aluminum sulphite is found 
sometimes on films or prints. This is caused by: (1) insuffi- 
_ cient rinsing after development; (2) too low a concentration 
of acid in the fixing bath; (3) insufficient agitation of the film 
on first immersing in the fixing bath. Since aluminum sul- 
phite is soluble in alkali, the scum may be removed by swab- 
bing the film or print with a 10% solution of sodium carbon- 
ate and then washing thoroughly. | 

With chrome alum fixing baths, a scum composed of | 
chromium hydroxide is produced as described under the 
section on “The Importance of Rinsing,” page 87. Films 
which are fixed in such a bath should always be wiped care- 
fully with wetted cotton for if any chromium scum dries on 
the surface it is impossible to remove it. A chrome alum 
fixing bath is not recommended for use with papers because 
of its slight staining characteristics. 

F. Stains. Several different types of stains such as 


white aluminum sulphite stain (see E above), sulphur stains, _ 


and yellow silver stains, are occasionally produced. For a 
complete discussion of fixing bath stains reference should be 
made to the article on this subject obtainable from the Service 
Department, Eastman Kodak Company, Rochester, N. Y, 

G. Moittle. When processing film or plates in hangers, — 
a mottled image is occasionally found when the hanger has ~ 
not been agitated enough on first immersing in the fixing bath, — 
or if the film is insufficiently rinsed between development and 


fixation. In the absence of thorough rinsing and agitation, 


development continues locally during the first few minutes 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY 95 


of fixing and in these spots the image has greater density. 
Mottle is also produced if the ends of the hanger protrude 
above the surface of the fixing bath, especially during the first 
stages of fixation. 


Effect of Temperature on Chemicals and Solutions 


Nearly all chemicals used in photographic work have a 
high enough melting point so that there is little danger of 
the solids melting while stored in bottles or other containers, 
but it is good practice to keep storage bottles as dry and cool 
as possible. Bottles or cans, for instance, should not be 
placed on shelves or in cupboards near a stove or where direct 
sunlight can shine upon them. 


When in solution the effect of temperature on chemicals 
is much greater and must be taken into account with every 
photographic solution. Under normal working conditions, 
a temperature of 65° F. (18° C.) is recommended for negative 
development, and 70° F. (21° C.) for print development. 


Temperatures of solutions are measured either by the 
Centigrade or Fahrenheit thermometer. On the Centigrade 
thermometer water freezes at zero and boils at Ioo degrees, 
and on the Fahrenheit scale the corresponding readings are 
32 degrees and 212 degrees, so that 100 degrees C. are equiva- 
lent to 212 degrees minus 32 degrees or 180 degrees F., that ts, 
1 degree C. is equivalent to 9/5 degrees F. 


To convert degrees Centigrade to Fahrenheit, multiply by 
9/5 and add 32. Toconvert degrees Fahrenheit to Centigrade 
subtract 32 and divide by 9/¢. 


Most chemical reactions proceed more rapidly as the tem- 
perature is increased, and this is true of all the reactions 
involved in photography, so that developers and fixing baths 
will act much more rapidly when warm than when cold. 
Different reactions are stimulated to different extents by rise 
of temperature, and the effect of temperature can be mea- 
sured numerically, the result obtained being termed the “‘tem- 
perature coefficient” of the reaction. 


As a general rule, the temperature coefficient is measured 
for a change of temperature of 10 degrees Centigrade, equiva- 
lent to 18 degrees Fahrenheit. Therefore, if a reaction takes 4 
minutes at 60° F. (15° C.) and is completed in 2 minutes at 
78° F. (25° C.) it is said to have a temperature coefficient of 2, 
the rate of reaction being doubled for a rise of 18° F. (10° C.). 


96 EASTMAN KODAK COMPANY 


The temperature coefficient of development varies with 
the developing agent, being least with the developers of 
high reduction potential, such as Elon, and most with 
developers of low reduction potential, such as hydroquinone. 
There is one consequence of this which is rather important. 
namely, that the behavior of a mixed hydroquinone developer — 
depends upon the temperature. At low temperature the hy- 
droquinone is very inert, while the Elon is not decreased in its 
rate of action to the same extent, and consequently the devel- 
oper behaves as if it contained an excess of Elon. At high 
temperatures the hydroquinone is increased in its activity far 
more than the Elon, and the situation is reversed. 


A similar principle applies to the fogging produced by 
developers. If development is continued for a sufficient time 
all developers will fog, but the fog reaction is a different one to 
that of development, and apparently has a different tempera- 
ture coefficient and one which is much higher than the tem- 
perature coefficient of the development reaction itself. Con- 
sequently, a developer which will develop a material to a good 
density with low fog at a normal temperature, may produce 
very bad fog if the temperature is high. 


From the above it will be understood that the control of — 
temperature in photography is of great importance and that — 
so far as possible development and fixation should always be ~ 
carried out at a normal temperature (65° to 70° F.) (18° to — 
21° C.), a serious change in temperature involving much 
greater care and the risk of difficulty. If the temperature is 
too high, then trouble may be encountered with fog and with 
softening and frilling of the material, while if the temperature 
is too low, development will be delayed, there is danger of 
under-development, and fixing will be slow so that the great- 
est care must be taken to insure thorough fixation. 


Fixing baths frequently decompose very rapidly with 
liberation of free sulphur if kept for a few hours to a few days 
at temperatures over 95°F. (35°C.). Although the rate of 
fixation is increased at higher temperatures, it is very bad 
practice to allow the temperature of a fixing bath to rise above 
75°F. (24° C.); the recommended temperature being 65° F. 
(18°C.). In storing large volumes of fixing bath it is best to 
store the hypo and hardener solutions separately and add the 


cool hardener to the cool hypo when the latter is put intoa _ 


working tank. 


Some strongly oxidizing solutions, acid permanganate, 


in particular, when employed at high. temperatures rapidly 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY — 97 


lose their effectiveness for photographic use owing to second- 
ary reactions. Usually these solutions work best at tempera- 
tures below 7o° F. (21°C.). 


Some photographic solutions, notably a hypo alum toning 
bath, are recommended to be used at 120° to 130° F. (49° to 
54° C.), but even these solutions should be watched carefully 
to see that the temperature does not rise above that recom- 
mended, otherwise blistering, staining, and degradation of 
tone will result. 


About the best general rule for temperature is to mix, 
store, and use the solutions at the temperature recommended 
in the manufacturer’s instructions. 


When the temperature cannot be controlled, as may be 
the case in the tropics, special measures must be taken as de- 
scribed completely in the booklet, “Tropical Development” 
obtainable on request from the Service Department, Eastman 


Kodak Company, Rochester, N. Y. 


Cleaning Containers for Photographic Solutions 


Apparatus used for mixing and containing photographic 
chemicals become discolored and sometimes coated with de- 
composition products of the solution. In certain cases this 
does no harm, especially if the container is always used for the 
same kind of solution, but it is much better technic to clean 
all containers each time they are emptied. With cheaper con- 
tainers, such as bottles and old trays, it is not worth wasting 
time if these are difficult to clean. It is better to discard the 
container and use a new one. 


Most cleaning solutions are either strong alkalis or acids, 
and should be used with the same discretion given these chem- 
icals when mixing photographic solutions. The principle of a 
cleaning solution is that it acts on the stain or deposit and 
changes it to a soluble form, which dissolves in the cleaning 
agent, or may be washed out with water. Sometimes the 
cleaning agent merely softens the deposit sufficiently so that 
it may be removed by the use of a wire brush or an abrading 
substance like sand or glass ‘beads. 


The most common tray cleaner is an acid solution of po- 
tassium bichromate made by dissolving 3 ounces (go grams) 
of potassium bichromate in 32 ounces (1 liter) of water, and 
adding slowly with stirring, 314 ounces (100 cc.) of concen- 
trated sulphuric acid. This solution will remove stains, 


98 EASTMAN KODAK COMPANY 


caused by oxidation products of developers, silver stains, and 
some dye stains, and is a very useful cleaning agent. 


Other dolutions which will be found useful are 1% per- 
manganate, (followed by treatment in 50% bisulphite to re- 
move the residual brown manganese stain); 40% sodium hy- 
droxide (caustic soda); and any of the strong mineral acids, 
such as sulphuric, hydrochloric, and nitric. After removal of 
the stain, the vessel should be washed thoroughly to insure 
complete ‘removal of the cleaning agent. 


When an acid fixing bath sulphurizes, the colloidal sul- 
phur is quite difficult to remove with a cleaning agent, but the 
addition of glass beads or sand to the bottle or other vessel 
with shaking will be found effective. A hot, concentrated 
solution of sodium sulphite (about 207%) will also usually dis- 
solve sulphur. 

Enamelled trays or tanks which have been used with 
strongly alkaline developers or caustic solutions, rapidly lose 
their glossy surface and become roughened and discolor easi- 
ly. If such containers are used with dye solutions, the dye is 
taken up in the pores of the enamel, and it is a very difficult 
matter to remove it completely. Trays badly discolored in 
this way are not worth the time to clean them. 


Large tanks of wood, Alberene, or stoneware, after several 
weeks of use as developer containers, become coated with a 
layer of slime or mold which should be cleaned out by thor- 
oughly scrubbing the walls with a wire brush, and then treat- 
ing with sodium hypochlorite solution. The solution whine 
be added in the following proportion: 1 part of 10% hy 
chlorite solution to 6 parts of water. After the solution 
been left in the tank overnight it should be emptied out iat 
the tank given another thorough scrubbing and several wash- 
ings previous to being put into service again. A stock solution 
of hypochlorite is prepared by making up a 4% solution of 
calcium hypochlorite and converting this into sodium 
hypochlorite. To prepare this solution, sodium carbonate 
solution (10%) is added to the calcium hypochlorite solution 
until no more precipitate forms, and the solution is then 
allowed to stand until all the precipitate settles to the bottom 
of the container. The remaining liquid is then drawn off for — 
use as a stock solution. 


ELEMENTARY PHOTOGRAPHIC CHEMISTRY — 99 


Table of Chemical Solubilities 


The following table will serve as a guide when preparing 
stock solutions of photographic chemicals. Since a solution 
is apt to become cooled 1n winter to a temperature approxi- 
mating 40° F., it is not advisable to prepare a stock solution 
stronger than is indicated by the solubility of the chemical 


at this temperature. 
Ounces of chemical in 100 ozs. 


Substance (fluid) of Saturated Solution at 
40° F. (4.4° C.) 70° B. (21.1° G.) 

Acid, Acetic (any strength) Mixes in all proportions 
Acid, Citric 78 88 
Acid, Oxalic 71, 141, 
Acid, Tartaric (dextro) 73 78 
Acrol or Amidol (See Diaminophenol 

Hydrochloride) 
Alum, Ammonium 614 151% 

_ Alum, Iron 48 59 

Alum, Potassium 61,4 1144 
Alum, Potassium Chrome 1514 201, 
Amidol or Acrol (See Diaminophenol 

Hydrochloride) 
Ammonia Solution Mixes in all proportions 
Ammonium Bromide 52 57 
Ammonium Carbonate 26 31 
Ammonium Chloride 26 30 
Ammonium Iodide 104 109 
Ammonium Oxalate 234 514 
Ammonium Persulphate 52 62 
Ammonium Thiocyanate or 

Ammonium Sulphocyanide 62 73 
Ammonium Thiosulphate, anhydrous 83 88 
Borax (Sodium Tetraborate) 21, VAY 


Caustic Potash (See Potassium Hydroxide) 
Caustic Soda (See Sodium Hydroxide) 


Copper Sulphate, crystal 26 31 
Diaminophenol Hydrochloride (Acrol or 

Amidol) 2014 26 
Elon (Monomethyl para-amino phenol 

sulphate) 51,4 814 

- Ferrous Sulphate ' 29 41 

Formalin Mixes in all proportions 
Hydroquinone 4l4, 634 


Hypo (See Sodium Thiosulphate) 
Kodelon (See Para-aminophenol oxalate) 


Lead Acetate 31 47 
Lead Nitrate 391, 51 
Mercuric Chloride 4 614 
Para-aminophenol oxalate (Kodelon) 14 214 
Potassium Bichromate 634 1414 
Potassium Bromide 50 56 
Potassium Carbonate, anhydrous 83 85 


Potassium Chloride 26 31 


100 EASTMAN KODAK COMPANY 


Substance 


Potassium Citrate 

Potassium Cyanide 

Potassium Ferricyanide 

Potassium Ferrocyanide 

Potassium Hydroxide (Caustic Potash) 
Potassium Iodide 

Potassium Metabisulphite 
Potassium Oxalate 

Potassium Permanganate 
Pyrogallol (Pyro) 

Silver Nitrate 

Sodium Acetate, anhydrous 

Sodium Acetate, crystal (trihydrate) 
Sodium Bicarbonate 

Sodium Bisulphite 

Sodium Bromide 

Sodium Carbonate, anhydrous 
Sodium Carbonate, crystal 

Sodium Chloride 

Sodium Hydroxide (Caustic Soda) 
Sodium Phosphate, dibasic crystal 
Sodium Sulphate, anhydrous 
Sodium Sulphate, crystal 

Sodium Sulphide, fused 

Sodium Sulphide, crystal 

Sodium Sulphite, anhydrous 
Sodium Tetraborate (See Borax) 
Sodium Thiosulphate, (Hypo) crystal 
Uranyl (Uranium) Nitrate 


Ounces of chemical in 100 ozs. 
(fluid) of Saturated Solution at 
40° F. (4.4° C.) 70° F. (21.1° C.) 


93 104 
46 52 
30 36 
17%, 26 
78 83 
99 104 
47 57 
29 361% 

3, 63, 
36 57 

109 135 
31 36 
52 62 

71, LVA 
52 52 
67 73 
10/4 24 
29 65 
31 31 
50 83 

614 24 

5, Ys 
1014 41 
1314 173, 
3614 47 
1714 28 
73 93 

114 130 


EASTMAN KODAK COMPANY 


10-28-CH-5 


RocHEsTER, NEw YORK 


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